Human Health and Disease
For most of human history, health was explained by ideas of balance — the Greek doctrine of four humors, or an Ayurvedic reading of temperament and bile. It took William Harvey's demonstration of blood circulation and a simple thermometer disproving the "hot-blooded" theory of fever to move medicine from reflective guesswork to evidence. Today biology defines health precisely: a state of complete physical, mental and social well-being, not merely the absence of disease — and explains disease as the outcome of three forces, genetic disorders present from birth, infections by external agents, and lifestyle choices around diet, hygiene, rest and habits. This chapter surveys that landscape system by system. It begins with the pathogens themselves — bacteria, viruses, protozoans, helminths and fungi — tracing how organisms as different as Salmonella typhi and the malarial parasite Plasmodium each find their own route into the human body and their own way of causing harm, and what public-health measures interrupt each route. It then turns inward, to the immune system that defends against all of them: the non-specific innate barriers present at birth, and the pathogen-specific acquired immunity built on the memory of B and T lymphocytes, which is also the biological basis of vaccination. Two of the gravest failures of this system anchor the second half of the chapter — AIDS, in which HIV deliberately dismantles the very helper T-cells that acquired immunity depends on, and cancer, in which a single cell escapes the normal controls on growth and division to become a self-perpetuating, invasive population. The chapter closes on a very different threat to health, rooted in behaviour rather than pathogens: the biology and psychology of drug and alcohol abuse in adolescence, and the addiction-dependence cycle that makes it so hard to escape. The unifying thread is that health is never automatic — it is actively defended, actively maintained, and actively undermined, and understanding the mechanisms in each case is what allows prevention to work.
- Health is complete physical, mental and social well-being, not simply the absence of disease; it is shaped by genetic disorders, infections and lifestyle.
- Pathogens span five groups — bacteria (typhoid, pneumonia), viruses (common cold), protozoans (malaria, amoebiasis), helminths (ascariasis, filariasis) and fungi (ringworm) — each with its own transmission route and control measure.
- Innate immunity is non-specific and present from birth: physical, physiological, cellular and cytokine barriers.
- Acquired immunity is pathogen-specific and has memory — a slow primary response is followed by a fast secondary (anamnestic) response — carried out by B-lymphocytes (humoral, antibody-mediated) and T-lymphocytes (cell-mediated immunity, CMI).
- Antibodies are H2L2 proteins of five classes (IgA, IgM, IgE, IgG, IgD); vaccination exploits immunological memory by pre-exposing the body to antigens.
- AIDS is caused by HIV, a retrovirus that uses reverse transcriptase, replicates inside macrophages and helper T-lymphocytes, and progressively destroys TH cells until opportunistic infections become fatal.
- Cancer arises when a cell loses contact inhibition; malignant tumors additionally invade tissue and metastasise via blood, unlike confined benign tumors.
- Drug and alcohol abuse centres on opioids (Papaver somniferum), cannabinoids (Cannabis sativa) and coca alkaloids (Erythroxylum coca), plus tobacco; adolescence is a vulnerable window for the addiction-dependence cycle to begin.
- Key idea: the body is defended by layered innate and pathogen-specific, memory-based acquired immunity, but that defence can itself fail catastrophically — through HIV's destruction of helper T cells, a cell's escape from growth control in cancer, or the addictive hijacking of the nervous system by abused drugs.
Human health rests on interrupting each pathogen's route of transmission, on layered innate barriers plus memory-based acquired immunity (humoral and cell-mediated), and is threatened systemically by HIV's destruction of helper T-lymphocytes, by cancer cells' escape from growth control into invasive, metastatic tumors, and by the addiction-dependence cycle of abused drugs and alcohol.
What this chapter covers
The chapter moves outward-in, then to two case studies. It opens with common diseases in humans (7.1): representative pathogens from bacteria, viruses, protozoans, helminths and fungi — their symptoms, transmission and control. It then examines immunity (7.2): innate and acquired immunity, antibody structure, active versus passive immunity, vaccination, allergies, autoimmunity, and the lymphoid organs. Two case studies follow: AIDS (7.3), where a virus specifically cripples acquired immunity, and cancer (7.4), where the normal controls on cell growth break down. The chapter ends with drug and alcohol abuse (7.5): the major abused substances, why adolescence is vulnerable, the biology of addiction and dependence, and prevention.
The core ideas
Defining health and classifying pathogens
Health is complete physical, mental and social well-being — not merely absence of disease or fitness. It depends on balanced diet, personal hygiene, regular exercise, disease awareness, vaccination, waste disposal, vector control and clean food and water. When an organ or system functions abnormally, producing signs and symptoms, the result is disease — either infectious (easily transmitted between people) or non-infectious (cancer is the major non-infectious killer). Disease-causing organisms are called pathogens, spanning bacteria, viruses, fungi, protozoans and helminths. Because pathogens live in or on a host and harm it, most parasites are pathogens; each must adapt to its host's specific environment, such as a gut pathogen surviving stomach acid and digestive enzymes.
Bacterial, viral and protozoan diseases
Typhoid (Salmonella typhi, bacterial) enters via contaminated food and water, migrates through blood, and causes sustained high fever (39-40°C), weakness, stomach pain, constipation and headache, confirmed by the Widal test. "Typhoid Mary," an asymptomatic carrier who spread it for years through food she cooked, shows how carriers transmit disease without symptoms. Pneumonia (Streptococcus pneumoniae, Haemophilus influenzae) infects the alveoli, which fill with fluid; symptoms are fever, chills, cough and headache, spread by inhaled droplets or shared utensils. The common cold (rhinoviruses) infects the nose and respiratory passage, not the lungs, causing nasal congestion, sore throat and cough for three to seven days, spread by droplets and contaminated objects like pens or doorknobs.
Malaria (Plasmodium; P. falciparum is most severe) enters as sporozoites through an infected female Anopheles bite, multiplies in liver cells, then invades and ruptures red blood cells, releasing the toxin haemozoin that causes fever recurring every three to four days. The parasite needs two hosts, human and mosquito, traced in the derivation below. Amoebiasis (Entamoeba histolytica) lives in the large intestine, causing constipation, cramps and blood-streaked stools; houseflies mechanically carry it from faeces to food.
Helminthic and fungal diseases
Ascariasis (Ascaris, roundworm) causes internal bleeding, muscular pain, fever, anaemia and intestinal blockage; eggs excreted in faeces contaminate soil, water and vegetables. Filariasis / elephantiasis (Wuchereria bancrofti, W. malayi) causes slow, chronic inflammation of the lymphatic vessels, usually of the lower limbs, spread by mosquito bites. Ringworm (Microsporum, Trichophyton, Epidermophyton) produces dry, scaly, itchy lesions on skin, nails and scalp, thriving in warm, moist skin folds, spread via soil or shared towels and combs.
| pathogen group | organism | disease | transmission |
|---|---|---|---|
| bacteria | Salmonella typhi | typhoid | contaminated food and water |
| bacteria | Streptococcus pneumoniae, Haemophilus influenzae | pneumonia | droplets, shared utensils |
| virus | rhinovirus | common cold | droplets, contaminated objects |
| protozoan | Plasmodium | malaria | Anopheles mosquito bite |
| protozoan | Entamoeba histolytica | amoebiasis | faecally contaminated food/water |
| helminth | Ascaris | ascariasis | contaminated water, vegetables |
| helminth | Wuchereria | filariasis | mosquito bite |
| fungus | Microsporum, Trichophyton, Epidermophyton | ringworm | soil, shared towels/combs |
Prevention. Personal hygiene (clean body, water, food) and public hygiene (waste disposal, water-reservoir disinfection, hygienic catering) prevent food- and water-borne diseases. Air-borne diseases additionally need avoiding close contact with infected persons; vector-borne diseases need breeding-site elimination — no stagnant water, mosquito nets, larvivorous fish like Gambusia, insecticide spraying, wire mesh on doors and windows — increasingly urgent given Aedes-borne dengue and chikungunya. Vaccination eradicated smallpox and sharply reduced polio, diphtheria and tetanus.
Innate immunity — non-specific defence
Immunity, the host's overall ability to fight pathogens, is innate or acquired. Innate immunity is non-specific and present from birth, built of four barriers: physical (skin; mucus lining the respiratory, gastrointestinal and urogenital tracts), physiological (stomach acid, saliva, tears), cellular (phagocytic leukocytes — polymorphonuclear neutrophils, monocytes, natural killer cells, tissue macrophages), and cytokine (interferons, secreted by virus-infected cells, protecting neighbouring uninfected cells).
Acquired immunity — specific and remembered
Acquired immunity is pathogen-specific and has memory: a low-intensity primary response on first exposure is followed by an intensified secondary (anamnestic) response on re-exposure to the same pathogen (traced in the first derivation below). B-lymphocytes produce antibodies released into blood — the humoral immune response — while T-lymphocytes do not secrete antibodies but help B cells and separately mediate cell-mediated immunity (CMI), responsible for graft rejection, which is why tissue and blood-group matching plus lifelong immunosuppressants are needed for transplants. Each antibody has four peptide chains, two light and two heavy, written ; five classes exist — IgA, IgM, IgE, IgG, IgD.
Active and passive immunity, vaccination, allergies and autoimmunity
When the host's own body produces antibodies against antigens it is exposed to, this is active immunity — slow to build but long-lasting, induced by infection or immunisation. When ready-made antibodies are given directly, this is passive immunity — colostrum's IgA and placental antibody transfer to a foetus are natural examples; antitoxin injections for tetanus or snakebite are deliberate ones. Vaccination exploits memory: a vaccine (antigenic proteins or a weakened/inactivated pathogen) triggers antibody production and memory B/T cells that respond overwhelmingly on real infection. Recombinant DNA technology now produces vaccines like hepatitis B from yeast. An allergy is an exaggerated immune response to environmental allergens (dust mites, pollen, dander), mediated by IgE and mast-cell histamine/serotonin release, treated with anti-histamines, adrenalin or steroids. When the body attacks its own cells, the result is autoimmune disease — rheumatoid arthritis is a common example.
The lymphoid organ system
Primary lymphoid organs — bone marrow (source of all blood cells) and thymus (large at birth, shrinking by puberty) — are where lymphocytes mature. Secondary lymphoid organs — spleen (filters blood, stores erythrocytes), lymph nodes (trap antigens in lymph, triggering local activation), tonsils, Peyer's patches and appendix — are where mature lymphocytes meet antigen and become effector cells. Mucosa-associated lymphoid tissue (MALT), lining the respiratory, digestive and urogenital tracts, makes up about half the body's lymphoid tissue.
AIDS — the immune system's own defenders destroyed
AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV, a retrovirus with an RNA genome, first reported in 1981 and responsible for over 25 million deaths. It spreads through sexual contact, contaminated blood/blood products, shared needles, and mother-to-child placental transmission — never by touch or ordinary contact. Symptoms typically appear five to ten years after infection. HIV infects macrophages, where reverse transcriptase converts its RNA to DNA that integrates into the host genome, turning macrophages into virus factories; it simultaneously infects helper T-lymphocytes (TH), progressively depleting them (traced in the third derivation below) until opportunistic infections — by Mycobacterium, fungi, and Toxoplasma — become fatal. Diagnosis uses ELISA; anti-retroviral drugs only prolong life. In India, NACO and NGOs lead prevention: safe blood banks, disposable needles, condom distribution and safe-sex education.
Cancer — the breakdown of growth control
Cancer, a major non-infectious killer, arises when cells lose contact inhibition — the property by which contact with neighbours halts uncontrolled growth — and divide indefinitely, forming tumors. Benign tumors stay confined and cause little damage; malignant tumors are rapidly proliferating neoplastic cells that invade surrounding tissue, starve normal cells of nutrients, and can metastasise — slough off, travel via blood, and seed new tumors at distant sites, the most feared malignant property.
| feature | benign tumor | malignant tumor |
|---|---|---|
| spread | stays at original site | invades and metastasises via blood |
| damage | relatively little | severe, competes for nutrients |
Carcinogens (physical, chemical or biological agents) transform normal cells into cancerous ones: ionising radiation and UV damage DNA, tobacco-smoke chemicals cause lung cancer, oncogenic viruses carry viral oncogenes, and normal proto-oncogenes (c-onc), when abnormally activated, drive transformation. Diagnosis uses biopsy with histopathology, blood/bone-marrow counts for leukaemias, and imaging — radiography, CT (X-ray based 3-D imaging) and MRI (magnetic fields, non-ionising). Treatment combines surgery, radiotherapy (lethal irradiation sparing normal tissue) and chemotherapy, plus immunotherapy using biological response modifiers such as alpha-interferon to activate the immune system against tumor cells that evade detection.
Drugs and alcohol abuse
Commonly abused drugs — opioids, cannabinoids, coca alkaloids — are mostly plant-derived. Opioids bind CNS and gut opioid receptors; heroin (diacetylmorphine) is made by acetylating morphine, extracted from the latex of opium poppy Papaver somniferum, and acts as a depressant; morphine itself is a clinically valuable sedative and painkiller. Cannabinoids, from the inflorescences of Cannabis sativa (marijuana, hashish, charas, ganja), act on brain cannabinoid receptors, affecting the cardiovascular system. Cocaine, from Erythroxylum coca, interferes with dopamine transport, producing stimulant euphoria and, in excess, hallucinations; Atropa belladonna and Datura are other hallucinogenic plants. Medically useful barbiturates, amphetamines and benzodiazepines are also abused. Tobacco's nicotine stimulates adrenaline/nor-adrenaline release, raising blood pressure and heart rate, and is linked to lung, bladder, throat and oral cancers, bronchitis and heart disease; smoking also raises blood carbon monoxide, lowering oxygen delivery.
Adolescence (roughly 12-18 years), a biologically and behaviourally transitional bridge to adulthood, is a vulnerable window: curiosity, academic stress, peer pressure, media portrayal of drug use as "cool," and unsupportive family structures are documented triggers. Addiction is psychological attachment to a drug's perceived benefits, deepened as rising receptor tolerance demands higher doses; dependence is the physical withdrawal syndrome — anxiety, shakiness, nausea, sweating — appearing when regular use stops abruptly, sometimes severely enough to need medical supervision. Abuse effects range from reckless behaviour and academic decline to coma, organ failure, and — via shared needles — HIV and Hepatitis B transmission; anabolic-steroid misuse by athletes causes masculinisation effects in females and reduced fertility, breast enlargement and stunted growth in adolescents of either sex. Prevention rests on respecting a child's own choices, education and counselling, seeking help from parents and trusted peers, watching for warning signs, and professional de-addiction and rehabilitation programmes.
The goal is to trace, stage by stage, why the malarial parasite needs two different hosts to complete a single life cycle, and to connect each stage to the specific symptom it produces — the periodic fever that is malaria's clinical signature. Seeing the cycle as a chain of obligatory stages explains both the recurring bouts of fever and why interrupting the mosquito stage is the most effective public-health lever against the disease.
Step 1. An infected female Anopheles mosquito bites a human and injects sporozoites, the infectious form of Plasmodium, from its salivary glands into the bloodstream — the only route of human infection.
Step 2. Sporozoites travel to the liver and multiply asexually within liver cells. This stage is clinically silent, since no red blood cells have yet been attacked.
Step 3. Parasites released from the liver invade red blood cells and multiply inside them; mature infected RBCs rupture synchronously, releasing fresh parasites and the toxin haemozoin. This synchronous release triggers the chill and high fever recurring every three to four days.
Step 4. Some parasites develop into sexual forms called gametocytes instead of continuing the asexual cycle. Gametocytes circulate in human blood but can develop no further there — a dead end unless taken up by a mosquito.
Step 5. A female Anopheles biting the infected person draws up gametocyte-laden blood. Inside the mosquito's gut, the gametocytes undergo sexual reproduction, ultimately producing sporozoites stored in the salivary glands.
Step 6. The cycle closes when this mosquito bites another human, restarting Step 1. Because the sexual stage occurs only in the mosquito, the mosquito is not merely a carrier but a biologically obligatory host, which is why eliminating mosquito breeding sites interrupts the disease at its most vulnerable link.
Go deeper · Derivation B · 1 minPrimary and secondary immune response — how immunological memory builds antibody levels
The goal is to explain, as a causal chain, why a first exposure to a pathogen produces only a weak, slow antibody response while every later exposure to the same pathogen produces a fast, powerful one — the mechanistic basis of natural immunity and of vaccination. The immune system does not just react to an antigen; it learns from the encounter and keeps a durable record.
Step 1. On first exposure, antigens on the pathogen's surface are recognised by a small number of matching B-lymphocytes among a vast, diverse pool, with T-lymphocyte help. Because few cells match at first, the initial response is limited.
Step 2. Matching B cells proliferate and differentiate into antibody-secreting plasma cells. This recruitment takes time, so the primary response rises slowly to a modest peak.
Step 3. A fraction of activated B and T lymphocytes become long-lived memory cells instead of short-lived plasma cells, persisting for years after the pathogen is cleared.
Step 4. On second exposure to the same pathogen, the large pre-existing memory-cell pool activates immediately. The resulting secondary (anamnestic) response rises faster, peaks far higher, and lasts longer than the primary response.
Step 5. Vaccination exploits exactly this: it deliberately triggers a primary response and memory-cell formation against a weakened or antigenic pathogen preparation without causing disease, so that a later real infection is met with an immediate, high-intensity secondary response — often fast enough to neutralise the pathogen before symptoms develop.
Go deeper · Derivation C · 1 minHow HIV progressively destroys the acquired immune system
The goal is to connect HIV's molecular replication to the clinical progression of AIDS — showing why a retrovirus that first infects macrophages ends up, years later, destroying the helper T-lymphocytes acquired immunity depends on, so ordinary infections turn fatal. Each step builds on the one before, turning a single infected cell into a systemic collapse of immunity.
Step 1. HIV enters the body through sexual contact, contaminated blood, shared needles, or placental transmission, and initially infects macrophages.
Step 2. Inside the macrophage, reverse transcriptase copies HIV's RNA genome into viral DNA — the defining reverse step of a retrovirus.
Step 3. This viral DNA integrates into the host macrophage's own DNA, reprogramming it to manufacture new HIV particles continuously while the cell itself survives — an ongoing "HIV factory."
Step 4. HIV also infects helper T-lymphocytes (TH) in parallel, replicating and releasing progeny virus that infects further TH cells, repeating the cycle.
Step 5. Because each cycle compromises more helper T cells than it started with, TH numbers fall progressively over five to ten years, with bouts of fever, diarrhoea and weight loss marking the decline.
Step 6. Helper T-lymphocytes coordinate acquired immunity — helping B cells make antibodies and activating other T cells. Once their numbers cross a critical threshold, the body cannot mount effective defence, and opportunistic infections by Mycobacterium, fungi and Toxoplasma become the immediate, usually fatal, outcome.
Worked examples
Board level - Differentiate between innate immunity and acquired immunity, with one example of each
Step 1. Identify the specificity criterion: innate immunity is non-specific and present from birth, defending generally against pathogens; acquired immunity is pathogen-specific, developing during life against particular pathogens.
Step 2. Identify the memory criterion: innate immunity shows no memory, responding identically every time; acquired immunity shows memory, giving a stronger, faster secondary response on repeat exposure.
Step 3. Give concrete examples: innate immunity — skin blocking microbial entry regardless of which microbe; acquired immunity — specific antibodies against measles virus after infection or vaccination, protecting only against measles.
Answer: Innate immunity is non-specific, present from birth, memory-less (example: skin as a physical barrier); acquired immunity is pathogen-specific, develops during life, and shows memory via a stronger secondary response (example: anti-measles antibodies).
Board level - Distinguish between a benign and a malignant tumor, and define metastasis
Step 1. Compare growth location: benign tumor cells stay confined to their original site; malignant tumor cells invade and damage surrounding normal tissue.
Step 2. Compare consequence: a benign tumor causes little damage since it does not spread; a malignant tumor's rapidly proliferating cells also compete with normal cells for nutrients, starving them.
Step 3. Define metastasis: cells slough off the primary malignant tumor, travel through blood, and lodge at a distant site, establishing a new secondary tumor — a property unique to malignant tumors.
Answer: A benign tumor stays confined and causes little damage; a malignant tumor invades tissue and starves normal cells. Metastasis is the spread of tumor cells via blood to distant sites, seeding new tumors — the most feared malignant property.
Competitive level (NEET) - Evaluate four statements on HIV and AIDS and identify the correct combination
Consider these statements. (I) HIV first infects helper T-lymphocytes and only later infects macrophages. (II) Reverse transcriptase converts the viral RNA genome into viral DNA inside the host cell. (III) HIV can be transmitted through ordinary physical contact such as shaking hands. (IV) The ELISA test diagnoses HIV infection. Which are correct?
Step 1. Statement I reverses the known order: HIV infects macrophages first, which become an ongoing viral factory, while helper T-lymphocytes are infected in parallel and progressively depleted — not infected "later." Statement I is false.
Step 2. Statement II matches the retrovirus mechanism exactly: reverse transcriptase copies HIV's RNA into DNA, which then integrates into the host genome. Statement II is true.
Step 3. Statement III contradicts the known transmission routes — HIV spreads only through body fluids (sexual contact, contaminated blood, shared needles, mother-to-child), explicitly not through touch. Statement III is false.
Step 4. Statement IV matches the named diagnostic test, ELISA. Statement IV is true.
Answer: Only statements II and IV are correct. Timing note: about 90 seconds. Resolve each statement independently against a specific chapter fact rather than judging the set as a whole — the trap is assuming a plausible-sounding statement (like I, which merely swaps an order) must be true. Anchor each check to a named mechanism: reverse transcriptase for II, the explicit "not spread by touch" line for III, the named test for IV.
Common traps
Innate and acquired immunity are often confused — only acquired immunity has memory and is pathogen-specific. Active and passive immunity are mixed up too — active means your own body makes antibodies (slow, lasting); passive means ready-made antibodies are given to you (fast, short-lived), as in colostrum or antitoxin injections. Humoral (B-cell, antibody-mediated) and cell-mediated (T-cell) immunity are distinct arms — CMI, not antibodies, drives graft rejection. Benign and malignant tumors are wrongly treated as a matter of size rather than behaviour — the defining difference is invasion and metastasis. Many think HIV directly kills the patient; in fact HIV destroys helper T-lymphocytes, and the resulting opportunistic infections are the usual immediate cause of death. Addiction and dependence are related but distinct: addiction is the psychological pull, dependence is the physical withdrawal syndrome.
You should be able to…
- Name a representative bacterial, viral, protozoan, helminthic and fungal pathogen, the disease each causes, and how each is transmitted.
- State the four barriers of innate immunity, with one example of each.
- Explain the primary versus secondary immune response and connect it to how vaccination works.
- Distinguish humoral (B-cell) immunity from cell-mediated (T-cell) immunity, and give one situation where each matters.
- Differentiate active immunity from passive immunity, with a natural example of each.
- Name the primary and secondary lymphoid organs and state the role of each.
- Trace how HIV infects macrophages and helper T-lymphocytes, and explain why opportunistic infections, not HIV directly, usually cause death in AIDS.
- Distinguish benign from malignant tumors and define metastasis and contact inhibition.
- Name the three major plant-derived drug classes, their plant sources, and the receptor systems they act on.
- Explain the difference between addiction and dependence, and list at least three prevention measures for drug and alcohol abuse.
Ask the tutor: Want to work through how a single HIV-infected macrophage leads, step by step, to the opportunistic infections that define full-blown AIDS?
If innate immunity is already present at birth, why do we need acquired immunity at all?
Innate immunity is fast but non-specific and memory-less — skin, mucus, stomach acid and phagocytes form a general first line of defence that treats every threat the same way every time. Many pathogens are specifically adapted to get past these general barriers or to survive once inside cells. Acquired immunity supplies what innate immunity lacks: a highly specific response tailored to each pathogen, plus memory that makes every later encounter far faster and more effective. The two work together — innate immunity buys time and contains infection early, while acquired immunity is built up to clear it thoroughly and remember it.
Why can a person get the common cold repeatedly but not measles a second time?
This comes down to how many distinct pathogens cause each disease. The common cold is caused by rhinoviruses, of which there are very many strains with different surface antigens; immunity to one strain does not protect against another, so repeated colds are expected. Measles is caused by a single, antigenically stable virus — after one infection or vaccination, memory B and T cells specific to it persist for years, so any later exposure triggers an immediate secondary response that clears the virus before symptoms develop. Acquired immunity is not failing against colds; colds simply involve many different targets while measles involves one.
Is HIV/AIDS transmitted by everyday contact like sharing a meal or shaking hands?
No. HIV transmits only through specific body fluids and routes — sexual contact, transfusion of contaminated blood or blood products, sharing contaminated needles, and mother-to-child placental transmission. It is not spread by touch, shared utensils, hugging or ordinary social contact, because the virus does not survive or transmit through skin contact or air. This matters medically and socially: isolating HIV-infected people from family or society is unnecessary and harmful to their well-being, and organisations like NACO specifically work to correct this misunderstanding.
Why are proto-oncogenes present in normal cells if they can cause cancer?
Proto-oncogenes (c-onc) are not inherently harmful — in their normal form they are ordinary genes regulating cell growth and division, processes every healthy cell needs. Cancer arises only when a proto-oncogene is abnormally activated, for instance by DNA damage from a carcinogen such as ionising radiation, UV light or tobacco-smoke chemicals, or by infection with an oncogenic virus carrying viral oncogenes. Once abnormally activated, the gene drives the cell's growth machinery out of control, causing loss of contact inhibition and runaway proliferation. The genes are a normal, necessary part of cell biology; it is their damage or inappropriate activation, not their mere presence, that causes cancer.
Once someone starts using drugs or alcohol, why is it so hard to stop even if they want to?
Two linked processes make this difficult. Repeated use raises the tolerance of relevant nervous-system receptors, so the same dose produces a weaker effect over time, pushing the user toward higher doses for the same perceived benefit — the addiction cycle. Once the body adapts to regular exposure, abruptly stopping produces genuine physiological withdrawal — dependence — with anxiety, shakiness, nausea and sweating severe enough, in some cases, to need medical supervision. Withdrawal symptoms are relieved by resuming use, strongly reinforcing continued use even against conscious intent. Because this is physiological, not simply willpower, professional counselling and medically supervised de-addiction programmes are usually necessary for lasting recovery.