Health and diseases caused by bacteria and viruses
Drinking untreated water or sharing a handkerchief with someone with a cold can pass on infections. Good health means much more than not being sick.
Health is a state of complete physical, mental and social well-being. A pathogen is a disease-causing organism. Typhoid (Salmonella typhi), pneumonia (Streptococcus pneumoniae, Haemophilus influenzae) and the common cold (rhinoviruses) are common infectious diseases.
In the figure : Each group of pathogens causes its own set of diseases.
Rule 1: Typhoid and pneumonia — Salmonella typhi enters the body through contaminated food and water, multiplies in the small intestine and spreads through blood to other organs. Symptoms: sustained high fever (39–40 °C), weakness, stomach pain, constipation, headache. It is confirmed by the Widal test. Pneumonia infects alveoli of the lungs, filling them with fluid; symptoms include fever, chills, cough and headache; the lips and fingernails may turn grey to bluish.
- Easy : Typhoid spreads through contaminated food and water.
- Medium : Mary Mallon ("Typhoid Mary") was a carrier who spread typhoid without showing symptoms.
- Exam-type : Q: Which test confirms typhoid? A: The Widal test.
Rule 2: Common cold — Rhinoviruses infect the nose and respiratory passage but not the lungs. Symptoms are nasal congestion, sore throat, cough, headache and tiredness for 3–7 days. It spreads by droplets from cough or sneeze and by contaminated objects such as pens and books.
- Easy : Cold affects the nose and throat.
- Medium : Washing hands and covering the mouth reduces its spread.
- Exam-type : Q: How does a common cold spread? A: Through droplets from infected persons and by contaminated objects.
Typhoid: Widal test. Pneumonia: alveoli filled with fluid. Common cold: rhinovirus, lungs not infected.
Writing that a cold affects the lungs. Rhinoviruses infect the upper respiratory tract, not the lungs.
Memory clue: S-T-W — Salmonella → Typhoid → Widal.
Spot the difference
| Point | Typhoid | Pneumonia |
|---|---|---|
| Pathogen | Salmonella typhi | Streptococcus pneumoniae, Haemophilus influenzae |
| Organ affected | Intestine, then blood | Alveoli of lungs |
| Test or sign | Widal test | Fluid in alveoli, grey–bluish lips |
Practice questions
- 1Define health.
- 2Name the pathogen of typhoid.
- 3Which part of the lung is infected in pneumonia?
- 4Name the virus group that causes the common cold.
- 5Which test is used to diagnose typhoid?
- 6How does typhoid spread?
- 7Why is Mary Mallon called a typhoid carrier?
- 8Why are cold and pneumonia treated differently?
Show answers (try them yourself first)
- 1A state of complete physical, mental and social well-being.
- 2Salmonella typhi.
- 3The alveoli.
- 4Rhinoviruses.
- 5The Widal test.
- 6Through contaminated food and water.
- 7She harboured the typhoid bacteria without symptoms and spread it to others.
- 8A common cold is a mild viral infection of the upper respiratory tract; pneumonia is a serious infection of the lungs.
Protozoan, helminth and fungal diseases
A mosquito bite can bring malaria; contaminated food can bring amoebic dysentery; damp skin can bring ringworm. Each has its own life cycle.
Malaria is caused by Plasmodium (P. vivax, P. malariae, P. falciparum) and is spread by the female Anopheles mosquito. Amoebiasis (Entamoeba histolytica), ascariasis (Ascaris), filariasis (Wuchereria) and ringworm (fungi Microsporum, Trichophyton, Epidermophyton) are other common diseases.
In the figure : The parasite needs both a human and a female Anopheles mosquito to complete its life cycle.
In the figure : Fever and chills appear when red blood cells rupture and release haemozoin.
Rule 1: Malaria — Sporozoites enter the blood through the mosquito's bite, go to the liver, multiply, attack red blood cells, and burst them, releasing a toxin called haemozoin that causes chill and high fever recurring every three to four days. P. falciparum causes the most serious malignant malaria, which can be fatal. In the mosquito gametocytes fuse and form sporozoites in the salivary glands.
- Easy : Malaria spreads by a female Anopheles.
- Medium : Haemozoin is a toxic substance.
- Exam-type : Q: Which stage of the parasite enters the human body? A: Sporozoites.
Rule 2: Other diseases — Amoebiasis (amoebic dysentery): constipation, abdominal pain, cramps, stools with excess mucus and blood clots; spread by houseflies carrying cysts. Ascariasis: internal bleeding, muscular pain, fever, anaemia and blockage of the intestinal passage; spreads by eggs in soil and water. Filariasis: lymphatic vessels of the lower limbs are blocked, causing swelling (elephantiasis). Ringworm: dry scaly lesions with intense itching, spread by soil or towels and clothes.
- Easy : Houseflies carry Entamoeba cysts.
- Medium : Elephantiasis is caused by Wuchereria.
- Exam-type : Q: Which fungi cause ringworm? A: Microsporum, Trichophyton and Epidermophyton.
Personal hygiene, clean food and water, waste disposal, vector control and vaccination prevent diseases. Dengue and chikungunya are spread by Aedes mosquitoes.
Thinking all mosquitoes spread malaria. Only the female Anopheles spreads it.
Memory clue: Anopheles → Malaria, Aedes → Dengue — A–M and A–D: Anopheles–Malaria, Aedes–Dengue.
Spot the difference
| Point | Malaria | Filariasis |
|---|---|---|
| Pathogen | Plasmodium (protozoan) | Wuchereria (worm) |
| Vector | Female Anopheles | Female mosquitoes |
| Sign | Fever and chills | Swelling of limbs |
Practice questions
- 1Name the vector of malaria.
- 2Which toxic substance causes fever in malaria?
- 3Name the pathogen of amoebiasis.
- 4What is the common name of Ascaris?
- 5Name two fungi that cause ringworm.
- 6Which mosquito spreads dengue?
- 7Why does malarial fever recur every three or four days?
- 8List two public health measures to avoid vector-borne diseases.
Show answers (try them yourself first)
- 1The female Anopheles mosquito.
- 2Haemozoin.
- 3Entamoeba histolytica.
- 4The common roundworm.
- 5Any two of Microsporum, Trichophyton, Epidermophyton.
- 6Aedes.
- 7RBCs burst at regular intervals, releasing haemozoin each time.
- 8Avoid stagnant water and use nets, repellents or insecticides.
Innate and acquired immunity
Your skin, tears and stomach acid work like a fort wall. If the wall is breached, special soldiers remember the intruder for years.
Immunity is the ability of the body to fight disease-causing organisms. Innate immunity is non-specific and present at birth. Acquired immunity is pathogen-specific and has memory: the first (primary) response is low; a second (secondary) response is much stronger.
In the figure : Interferons are cytokines made by virus-infected cells; they protect nearby cells.
In the figure : Both kinds of response are specific to the antigen.
Rule 1: Innate immunity — Physical barriers (skin, mucus lining of the respiratory, gastrointestinal and urogenital tracts); physiological barriers (gastric acid, saliva, tears); cellular barriers (neutrophils, monocytes, natural killer cells and macrophages that engulf germs by phagocytosis); cytokine barriers (virus-infected cells secrete interferons).
- Easy : Stomach acid is a physiological barrier.
- Medium : Neutrophils are phagocytes.
- Exam-type : Q: Name the cytokines that protect uninfected cells from virus. A: Interferons.
Rule 2: Acquired immunity — The lymphocytes B and T respond. B-cells produce antibodies (humoral immunity); the antibody molecule has four peptide chains (two heavy and two light, H₂L₂). Types of antibody: IgA, IgM, IgE, IgG. T-cells mediate cell-mediated immunity; this is why transplanted organs are rejected. Memory B and T cells give a quick, stronger secondary response.
- Easy : Antibody = H₂L₂ protein.
- Medium : T-cells cause graft rejection.
- Exam-type : Q: Why is the secondary response stronger? A: Memory cells recognise the antigen at once.
Innate = non-specific, no memory. Acquired = specific, has memory. Antibodies (immunoglobulins): IgA, IgM, IgE, IgG.
Thinking that T-cells make antibodies. Only B-cells (plasma cells) make antibodies.
Memory clue: B for Blood fluid, T for Tissue cells — B-cells = humoral (antibodies in body fluids); T-cells = cell-mediated.
Spot the difference
| Point | Innate immunity | Acquired immunity |
|---|---|---|
| Present | From birth | Developed after exposure |
| Specificity | Non-specific | Pathogen-specific |
| Memory | No | Yes |
Practice questions
- 1What is immunity?
- 2Give one physical barrier of innate immunity.
- 3Which cells produce antibodies?
- 4Name two types of antibodies.
- 5What are interferons?
- 6What kind of immunity do T-cells give?
- 7Why is a second infection by the same pathogen less severe?
- 8Why are organs from another person rejected without drugs?
Show answers (try them yourself first)
- 1The body's ability to fight disease-causing organisms.
- 2Skin (or mucus lining).
- 3B-lymphocytes (plasma cells).
- 4Any two of IgA, IgM, IgE, IgG.
- 5Cytokine proteins from virus-infected cells that protect other cells.
- 6Cell-mediated immunity.
- 7Memory cells give a quicker and stronger response.
- 8T-cells (cell-mediated response) recognise the foreign tissue and attack it.
Vaccination, allergy, autoimmunity and lymphoid organs
A single injection can protect a child for years from a disease. But the same immune system can sometimes overreact to dust or pollen.
In active immunity the body makes its own antibodies after exposure to an antigen (natural infection or vaccination); in passive immunity ready-made antibodies are given (mother's milk colostrum, antivenom). Allergy is an exaggerated response to an allergen, with IgE, histamine and serotonin. Autoimmunity is when the body attacks its own cells (rheumatoid arthritis).
In the figure : Vaccines use weakened or dead pathogens or their proteins to give memory.
In the figure : Immature lymphocytes mature in primary organs and meet antigens in secondary organs.
Rule 1: Vaccination — A vaccine contains inactivated or weakened pathogens (or their antigenic proteins). It produces antibodies and memory cells. If the real pathogen enters later, the memory cells give a quick response. Tetanus and snakebite are treated with ready-made antibodies (antitoxin, antivenom). Recombinant DNA technology produces vaccine proteins in yeast (hepatitis B vaccine).
- Easy : Vaccine builds memory cells.
- Medium : Snakebite is treated with antivenom antibodies, a passive treatment.
- Exam-type : Q: Why is a vaccine given before infection? A: It gives memory cells for a quick response later.
Rule 2: Allergy, autoimmunity and lymphoid organs — Allergens (pollen, dust mites) cause mast cells to release histamine and serotonin, giving sneezing, watery eyes and breathing trouble; antihistamines, adrenalin and steroids reduce the symptoms. The thymus (near the heart) shrinks with age. The spleen filters blood and stores red blood cells. Mucosal lymphoid tissue (MALT) lines the respiratory, digestive and urogenital tracts and makes up about 50 per cent of lymphoid tissue.
- Easy : Histamine causes allergy symptoms.
- Medium : Antihistamine drugs reduce allergy.
- Exam-type : Q: Which organs are primary lymphoid organs? A: Bone marrow and thymus.
Colostrum gives passive immunity (IgA) to the baby. Active immunity is slow but long-lasting; passive immunity is quick but short-lived.
Writing that a vaccine gives passive immunity. Vaccines give active immunity (the body makes antibodies).
Memory clue: A-Active → Actor (own), P-Passive → Present (given) — Active: your body works; Passive: antibodies are given to you.
Spot the difference
| Point | Allergy | Autoimmunity |
|---|---|---|
| Target | Harmless foreign substance | Body's own cells |
| Mediator | IgE, histamine | Autoantibodies, T-cells |
| Example | Hay fever | Rheumatoid arthritis |
Practice questions
- 1Define vaccination.
- 2Name one example of passive immunity.
- 3What do mast cells release during allergy?
- 4Give an example of an autoimmune disease.
- 5Name two primary lymphoid organs.
- 6Which organ filters blood and stores red cells?
- 7Why does a baby need the first milk of its mother?
- 8Why are antihistamines used when a person is allergic?
Show answers (try them yourself first)
- 1Giving a harmless antigen preparation to produce immunity.
- 2Colostrum (or antivenom).
- 3Histamine and serotonin.
- 4Rheumatoid arthritis.
- 5Bone marrow and thymus.
- 6The spleen.
- 7Colostrum has antibodies (IgA) that protect the baby.
- 8They block the effect of histamine, which causes allergy symptoms.
AIDS
A virus that hides inside the very cells meant to protect us makes AIDS a different kind of disease. Knowing how it spreads is the best protection.
AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV (human immunodeficiency virus), a retrovirus with an RNA genome. Inside the host cell its RNA is copied into DNA by reverse transcriptase and the DNA joins the host DNA. HIV infects helper T-cells and also macrophages, so immunity falls.
In the figure : Macrophages act as HIV factories; the number of helper T-cells decreases.
In the figure : Knowing the facts removes fear and social discrimination.
Rule 1: Course and diagnosis — There is a period of months to many years with no symptoms; then fever, diarrhoea and weight loss. As helper T-cells fall, the person cannot fight even minor infections (opportunistic infections). Diagnosis uses ELISA. Anti-retroviral drugs only partially help.
- Easy : HIV attacks helper T-cells.
- Medium : ELISA detects HIV.
- Exam-type : Q: What enzyme makes DNA from the HIV RNA? A: Reverse transcriptase.
Rule 2: Prevention — Use disposable or sterilised needles, screened blood, condoms; avoid multiple partners; give education and counselling. NACO (National AIDS Control Organisation) and NGOs run awareness programmes. Infected people must not be discriminated against.
- Easy : Use only screened blood.
- Medium : Use a new needle each time.
- Exam-type : Q: Name an Indian organisation working against AIDS. A: NACO.
HIV is a retrovirus (RNA → DNA). Helper T-cells fall; the body is exposed to opportunistic infections.
Thinking AIDS spreads by touch or sharing food. It spreads by blood, sexual contact and from mother to child.
Memory clue: R-T-H — Reverse Transcriptase, Helper T-cells.
Spot the difference
| Point | HIV | AIDS |
|---|---|---|
| Is | The virus | The disease (syndrome) |
| Genome | RNA (retrovirus) | Weak immunity |
| Test | ELISA detects infection | Many opportunistic infections |
Practice questions
- 1Expand AIDS.
- 2What is the genetic material of HIV?
- 3Which cells does HIV mainly infect?
- 4Name the test used to detect HIV.
- 5Which enzyme is present in a retrovirus?
- 6Give one way by which HIV spreads.
- 7Why do AIDS patients die of ordinary infections?
- 8Why should screened blood be used for transfusion?
Show answers (try them yourself first)
- 1Acquired Immuno Deficiency Syndrome.
- 2RNA.
- 3Helper T-lymphocytes (and macrophages).
- 4ELISA.
- 5Reverse transcriptase.
- 6Unprotected sex, infected blood or needles, mother to child.
- 7Without helper T-cells, immunity is too weak.
- 8To avoid transferring HIV and other viruses.
Cancer
Normal cells stop dividing when they touch each other. In cancer, cells ignore this signal and keep growing like a crowd that never stops pushing.
Cancer is uncontrolled cell division that forms a mass called a tumour. Normal cells show contact inhibition; cancer cells have lost it. Benign tumours stay confined; malignant tumours spread to other parts by metastasis. Agents that cause cancer are carcinogens (tobacco smoke, X-rays, UV rays, certain viruses).
In the figure : Metastasis is the most feared property of malignant tumours.
Rule 1: Causes — Physical (ionising radiation like X-rays, UV rays), chemical (tobacco smoke) and biological (oncogenic viruses) carcinogens damage DNA. Cellular oncogenes (proto-oncogenes) in normal cells, when activated, lead to cancer.
- Easy : Smoking can cause lung cancer.
- Medium : UV rays damage skin DNA.
- Exam-type : Q: What are proto-oncogenes? A: Normal cellular genes which when activated become oncogenes.
Rule 2: Detection and treatment — Detection: biopsy and histopathology, blood and bone marrow tests for leukaemia, CT and MRI scans, and antibodies against cancer-specific antigens. Treatment: surgery, radiotherapy, chemotherapy, and immunotherapy with biological response modifiers such as α-interferon. Cancer cells evade the immune system, so interferon helps the immune system to destroy the tumour.
- Easy : Biopsy examines a piece of tissue.
- Medium : Chemotherapy uses drugs that kill cancer cells.
- Exam-type : Q: What is biopsy? A: Taking a thin section of suspected tissue and studying it under a microscope.
Contact inhibition is lost in cancer cells. Metastasis = spread of cancer cells to distant sites through blood.
Thinking all tumours are cancer. Benign tumours do not spread.
Memory clue: BM — Benign = Mild (stays), Malignant = Moves (metastasis).
Spot the difference
| Point | Normal cell | Cancer cell |
|---|---|---|
| Contact inhibition | Present | Lost |
| Division | Controlled | Uncontrolled |
| Spread | No | Metastasis (in malignant) |
Practice questions
- 1What is a tumour?
- 2What is metastasis?
- 3Name one chemical carcinogen.
- 4What is contact inhibition?
- 5Name a physical carcinogen.
- 6What do you call a tumour that does not spread?
- 7How can leukaemia be detected?
- 8Why is early detection of cancer important?
Show answers (try them yourself first)
- 1A mass of cells formed by uncontrolled division.
- 2Spread of cancer cells to distant parts of the body.
- 3Tobacco smoke.
- 4Normal cells stop dividing when they touch each other.
- 5X-rays, gamma rays or UV rays.
- 6Benign tumour.
- 7By increased white blood cell count in a blood test and a bone marrow examination.
- 8Early cancers can be removed or treated before they spread.
Drugs and alcohol abuse
A student under exam stress or peer pressure may try a drug for relief. Knowing how addiction works helps one to say no in time.
Drugs are chemicals that change body functions. The major groups of drugs of abuse are opioids (heroin or smack from morphine, obtained from opium poppy Papaver somniferum), cannabinoids (marijuana, hashish, charas, ganja from Cannabis sativa) and coca alkaloids (cocaine from Erythroxylum coca). Tobacco, alcohol, LSD and others are also abused.
In the figure : Different drugs act on the nervous system in different ways.
In the figure : Addiction can be avoided by early professional help and counselling.
Rule 1: Addiction and dependence — Addiction is a psychological attachment to the euphoric effects of drugs; with time tolerance increases. Dependence is the body's tendency to show withdrawal syndrome (anxiety, shakiness, nausea, sweating) if the regular dose is suddenly stopped; this can be severe and even fatal.
- Easy : Withdrawal symptoms show dependence.
- Medium : Tolerance means a bigger dose is needed.
- Exam-type : Q: What is tolerance? A: Need for increasing doses to get the same effect.
Rule 2: Effects and prevention — Opioids bind to opioid receptors in the central nervous system and gastrointestinal tract. Cocaine acts on the nervous system, interferes with neurotransmitter dopamine transport; cannabinoids affect the cardiovascular system. Alcohol harms the liver (cirrhosis) and the nervous system; sharing needles spreads HIV and hepatitis B. Athletes who use anabolic steroids and other dope face health risks: mood swings, liver and kidney damage. Prevention: avoid undue peer pressure, seek help from parents and teachers, and get professional medical help; education and counselling.
- Easy : Alcohol abuse damages the liver.
- Medium : Sharing needles spreads HIV.
- Exam-type : Q: Name two ways of preventing drug abuse. A: Counselling and avoiding peer pressure (also seeking medical help early).
Opioids: heroin (smack) from opium poppy. Cannabinoids: from Cannabis sativa. Cocaine: from Erythroxylum coca. Nicotine in tobacco raises blood pressure and heart rate.
Thinking alcohol only affects the liver. It also affects the brain and behaviour, and causes road accidents.
Memory clue: O-C-C — Opioids, Cannabinoids, Coca alkaloids.
Spot the difference
| Point | Opioids | Cannabinoids |
|---|---|---|
| Source | Opium poppy | Cannabis sativa |
| Example | Heroin, morphine | Marijuana, hashish |
| Effect | Pain relief, depress nervous system | Affect cardiovascular system and perception |
Practice questions
- 1Name the plant source of morphine.
- 2What is another name for heroin?
- 3Which plant gives cocaine?
- 4What is meant by addiction?
- 5What are withdrawal symptoms?
- 6Name two harmful effects of alcohol.
- 7Why do students try drugs, and how can this be avoided?
- 8Why are anabolic steroids dangerous for athletes?
Show answers (try them yourself first)
- 1Opium poppy, Papaver somniferum.
- 2Smack (diacetylmorphine).
- 3Erythroxylum coca.
- 4Psychological attachment to the effects of a drug.
- 5Unpleasant effects when a regular drug dose is stopped suddenly.
- 6Liver damage (cirrhosis) and impaired judgement/nervous system.
- 7Because of curiosity, stress and peer pressure; by counselling, education and seeking help.
- 8They cause mood swings, aggression and liver and kidney damage.