What is health?
Health is not only 'no disease'. It means the body, the mind and our social life are all in good shape.
Three things keep us healthy: good genes, a clean place to live, and good habits (balanced food, exercise, sleep, clean hands).
A disease is when some organ or system stops working properly. An infectious disease spreads from one person to another (like cold). A non-infectious disease does not spread (like cancer).
Pathogens and common diseases
A pathogen is any living thing that causes disease. Pathogens can be bacteria, viruses, fungi, protozoa (one-celled animals) or worms.
| Disease | Pathogen | How it spreads | Main signs |
|---|---|---|---|
| Typhoid | Salmonella typhi (bacterium) | dirty food and water | long high fever, stomach pain, weakness; the gut wall can break. Confirmed by the Widal test. |
| Pneumonia | Streptococcus pneumoniae, Haemophilus influenzae (bacteria) | droplets from coughs and sneezes, shared cups | air sacs (alveoli) fill with fluid; fever, cough, breathing trouble; lips and nails may turn bluish |
| Common cold | rhino viruses | droplets, touching things | blocked nose, sore throat; lungs are not affected; lasts 3–7 days |
| Malaria | Plasmodium (protozoan): P. vivax, P. malariae, P. falciparum (most dangerous) | bite of female Anopheles mosquito | chills and high fever that come back every 3–4 days |
| Amoebiasis | Entamoeba histolytica (protozoan) | houseflies carry it to food | stomach pain, stool with mucus and blood |
| Ascariasis | Ascaris (roundworm) | eggs in soil, water, unwashed vegetables | stomach pain, anaemia; worms can block the intestine |
| Filariasis (elephantiasis) | Wuchereria (worm) | bite of female Culex mosquito | lymph vessels swell, legs look very thick |
| Ringworm | fungi: Microsporum, Trichophyton, Epidermophyton | towels, combs, moist sweaty skin | dry, itchy, round patches on skin, nails, scalp |
| Dengue and chikungunya | viruses | bite of Aedes mosquito (bites by day) | fever, strong joint pain, rash; dengue lowers platelets |
Life cycle of Plasmodium (malaria germ)
- An infected female Anopheles bites. Tiny forms called sporozoites go into our blood.
- They reach the liver and multiply there.
- They enter red blood cells and multiply again. The red cells burst. The burst releases a toxin called haemozoin. This causes the chill and high fever every 3–4 days.
- Some forms become gametocytes (sex forms) in the blood.
- A new mosquito sucks this blood. Inside the mosquito's gut, male and female gametes join. New sporozoites form and move to its salivary glands. Now this mosquito can infect the next person.
So Plasmodium needs two hosts: humans (asexual multiplication) and the mosquito (sexual stage).
How do we control these diseases?
- Personal hygiene: clean body, wash hands, drink clean (boiled or filtered) water, eat clean food.
- Public hygiene: proper disposal of waste and excreta, clean water tanks, clean toilets.
- Stop air spread: keep away from coughing people, cover coughs, do not share cups.
- Stop vectors (carriers like mosquitoes and flies): no still water near homes, empty coolers weekly, use nets and screens, spray where needed, put Gambusia fish in ponds (they eat mosquito larvae).
- Vaccines and medicines: many diseases like polio, diphtheria, tetanus, smallpox and pneumonia are controlled by vaccines; antibiotics treat many bacterial diseases.
Immunity: innate and acquired
Immunity is the body's power to fight germs. There are two kinds.
Innate immunity (born with it)
It works on every germ in the same way. It has no memory. It has four kinds of barriers:
- Physical: skin; mucus lining in nose, lungs, gut that traps germs.
- Physiological: stomach acid, saliva, tears.
- Cellular: white cells like neutrophils, monocytes and natural killer cells, and macrophages in tissues, eat or kill germs.
- Cytokine: cells infected by a virus release interferons, which protect nearby cells.
Acquired immunity (learned)
It is specific: it knows each germ. It has memory. The first meeting gives a slow, weak primary response. The next meeting with the same germ gives a fast, strong secondary (anamnestic) response.
Two kinds of cells do this: B lymphocytes make antibodies into the blood (humoral immunity). T lymphocytes help B cells and kill infected cells themselves (cell-mediated immunity). Cell-mediated immunity is why a transplanted organ can be rejected, so donors must be matched.
Antibody structure
An antibody is a Y-shaped protein. It has four chains: two long heavy chains and two short light chains. So it is written H2L2. The two tips of the Y grab the germ's marker (antigen). Types of antibodies: IgA, IgM, IgE, IgG (IgD too).
Active and passive immunity; vaccines
- Active immunity: our own body makes the antibodies, after an infection or a vaccine. It is slow to start but lasts long.
- Passive immunity: ready-made antibodies are given. Examples: colostrum (first yellow milk of the mother) has IgA for the baby; antibodies cross the placenta to the foetus; antitoxin injection after a snake bite or for tetanus. It works fast but does not last.
Vaccination: a weak, dead or inactivated germ (or its protein) is put in the body. B and T memory cells form. Later, the real germ is destroyed quickly. Immunisation means giving this protection. Using recombinant DNA, some vaccine proteins are now made in yeast or bacteria, like the hepatitis B vaccine.
Allergy, autoimmunity and the immune organs
- Allergy: the body over-reacts to harmless things (dust, pollen, mites). IgE antibodies are made. Mast cells release histamine and serotonin: sneezing, watery eyes, breathlessness. Medicines: anti-histamines, adrenaline, steroids.
- Autoimmunity: the body attacks its own cells. Example: rheumatoid arthritis.
- Primary lymphoid organs (where lymphocytes are made and mature): bone marrow and thymus.
- Secondary lymphoid organs (where lymphocytes meet germs): spleen, lymph nodes, tonsils, Peyer's patches in the small intestine, appendix, and MALT (mucosa-associated lymphoid tissue, about half the body's lymphoid tissue).
AIDS and HIV
AIDS = Acquired Immuno Deficiency Syndrome. 'Acquired' means we are not born with it. It is caused by HIV (Human Immunodeficiency Virus), a retrovirus (its genes are RNA).
How it spreads: unsafe sex with an infected person, infected blood, shared needles (as in drug users), and an infected mother to her baby. It does not spread by touching, hugging, sharing food, or mosquito bites.
How it works:
- HIV enters macrophages. Its RNA is copied into DNA by the enzyme reverse transcriptase. This DNA joins the cell's DNA. The macrophage becomes a 'virus factory'.
- HIV then enters helper T cells (TH), copies itself and kills them.
- TH cell numbers fall slowly over years. The person may have fever, diarrhoea and weight loss.
- When very few TH cells are left, even weak germs (like Mycobacterium, Toxoplasma, fungi) cause serious disease.
Test: ELISA. Treatment: anti-retroviral drugs slow it down but do not cure it. Prevention: safe blood, one-time needles, condoms, awareness. NACO (National AIDS Control Organisation) works on this in India. People with HIV need care and respect, not fear.
Cancer
Normal cells divide in a controlled way. When a normal cell touches other cells, it stops dividing. This is contact inhibition. Cancer cells have lost it. They keep dividing and make a lump called a tumour.
- Benign tumour: stays in one place, does less harm.
- Malignant tumour: grows fast, invades nearby tissue, and some cells travel through the blood to form new tumours far away. This spreading is metastasis.
Causes (carcinogens): X-rays and gamma rays (ionising radiation), UV rays (non-ionising), chemicals in tobacco smoke, and some viruses (oncogenic viruses). Cells carry proto-oncogenes (cellular oncogenes); when these are switched on wrongly, cancer can start.
Detection: biopsy (a thin tissue piece checked under a microscope), blood and bone marrow tests (for leukaemia), X-ray, CT scan, MRI, and antibodies against cancer markers.
Treatment: surgery, radiation therapy, chemotherapy (medicines that kill fast-dividing cells), and immunotherapy – for example α-interferon helps the immune system kill the tumour.
Drug and alcohol abuse in adolescence
Adolescence is roughly age 12 to 18: the body and mind change fast. Curiosity, peer pressure, stress about exams and the wish to look 'grown-up' can lead some teens to try tobacco, alcohol or drugs.
| Group | Source | Effect |
|---|---|---|
| Opioids (heroin/smack) | from morphine of the poppy plant Papaver somniferum | slow down the body (depressant); bind to opioid receptors in brain and gut |
| Cannabinoids (marijuana, hashish, charas, ganja) | Cannabis sativa | affect the heart and blood vessels |
| Cocaine (coke, crack) | coca plant Erythroxylum coca | block dopamine removal: false joy and energy; large doses cause hallucinations |
| Hallucinogens | Atropa belladonna, Datura | see or hear things that are not there |
| Tobacco | nicotine | raises blood pressure and heart rate; smoking causes lung, throat and bladder cancer; carbon monoxide lowers oxygen in blood |
Addiction means the body and mind need the drug more and more. Dependence means stopping causes painful withdrawal (anxiety, shaking, sweating). Alcohol damages the liver (cirrhosis) and nerves. Sharing needles spreads HIV and hepatitis B. Some athletes misuse steroids, which harm the body.
Warning signs: falling grades, missing school, losing interest in hobbies, secret behaviour, changes in sleep and food.
Prevention: avoid pressure to 'perform', learn to say no to peers, talk to parents, teachers and trusted friends, get help early from doctors and counsellors.
Try it: count the breeding spots
Look around your home and school. Count places where water stays still for a week: cooler trays, flower-pot plates, old tyres, water tanks without lids. Write the number. Empty or cover each one. Then check again after a week. This is exactly how dengue and malaria are controlled.
Key formulas and definitions
- Antibody = H₂L₂ (2 heavy + 2 light chains, Y shape)
- Innate immunity: born with, non-specific, no memory
- Acquired immunity: specific, has memory (primary slow → secondary fast)
- Humoral (B cells, antibodies) vs cell-mediated (T cells)
- HIV: RNA → DNA by reverse transcriptase; target = helper T cells
- Malaria: Plasmodium + female Anopheles; Dengue: virus + Aedes; Filariasis: Wuchereria + Culex
Worked examples
1. A child drinks water from a roadside tap and gets a long high fever and stomach pain. A blood test called the Widal test is positive. Name the disease and the germ.
Typhoid. The germ is the bacterium Salmonella typhi. It spreads by dirty water and food. The Widal test confirms it.
2. Why does a malaria patient get fever again and again every few days, and not all the time?
Plasmodium multiplies inside red blood cells. When many red cells burst together, a toxin called haemozoin is released. This gives chills and high fever. The next round of multiplication takes about 3–4 days, so the fever comes back in cycles.
3. A person gets measles once and never again, even when a sibling gets it later. Explain.
The first infection made memory B and T cells. On the second meeting, they give a fast, strong secondary response. The virus is killed before the person feels ill. This is active acquired immunity.
4. A snake-bite patient is given an injection of antivenom. Is this active or passive immunity? Why is it used instead of a vaccine?
Passive immunity, because ready-made antibodies are given. The poison acts in minutes, and the body would take days to make its own antibodies. So quick ready-made help is needed.
5. A person's helper T cell count fell from about 1000 to 150 per microlitre over eight years. What disease is likely, and why do ordinary infections now become dangerous?
HIV infection progressing to AIDS. Helper T cells guide both B cells and killer T cells. With very few of them, the immune system cannot respond, so weak germs like Toxoplasma or fungi cause serious illness.
6. A lump is found. The doctor says it has not spread and has a clear boundary. Another person's lump has sent cells to the lungs. Which is benign and which is malignant?
The first is a benign tumour (stays in place). The second is malignant, because its cells spread to another organ by metastasis.
Common mistakes
- Saying mosquitoes spread HIV. They do not. HIV spreads by unsafe sex, infected blood or needles, and mother to baby.
- Mixing up the mosquitoes: Anopheles = malaria, Aedes = dengue and chikungunya, Culex = filariasis.
- Thinking antibiotics cure cold or flu. These are viral diseases; antibiotics work only on bacteria.
- Calling colostrum or antitoxin 'active' immunity. Ready-made antibodies = passive immunity.