What is health?
Health is not just the absence of disease. It is a state of complete physical, mental and social well-being. Good health depends on a balanced diet, personal hygiene, regular exercise, awareness of diseases, vaccination, proper disposal of waste and control of disease vectors.
Diseases that spread from one person to another are infectious. Organisms that cause them are pathogens: bacteria, viruses, fungi, protozoa and helminths (worms).
Common infectious diseases
Disease | Pathogen | Spread by | Key symptoms |
|---|---|---|---|
Typhoid | bacterium Salmonella typhi | contaminated food and water | sustained high fever (39–40 °C), weakness, stomach pain; confirmed by the Widal test |
Pneumonia | bacteria Streptococcus pneumoniae, Haemophilus influenzae | droplets, shared utensils | alveoli fill with fluid; fever, chills, cough; lips and nails may turn bluish |
Common cold | rhinoviruses | droplets, contaminated objects | infects nose and respiratory passage, not lungs; nasal congestion, sore throat |
Malaria | protozoan Plasmodium (P. vivax, P. malariae, P. falciparum; falciparum is the most serious) | bite of the female *Anopheles* mosquito | chills and high fever recurring every 3–4 days |
Amoebiasis (amoebic dysentery) | protozoan Entamoeba histolytica | houseflies carry it to food; contaminated food and water | constipation, cramps, stools with mucus and blood |
Ascariasis | roundworm Ascaris | eggs in contaminated soil, water, vegetables | internal bleeding, muscle pain, anaemia, blockage of the intestine |
Filariasis (elephantiasis) | worm Wuchereria | bite of the female Culex mosquito | chronic swelling of the limbs and genital organs |
Ringworm | fungi (Microsporum, Trichophyton, Epidermophyton) | contact; shared towels and combs | dry, scaly, itchy lesions on skin, nails and scalp; thrives in warm, moist areas |
The malaria parasite's life cycle
Plasmodium needs two hosts: humans and the female Anopheles mosquito.
An infected mosquito bites a person and injects sporozoites, the infectious form.
Sporozoites reach the liver and multiply.
They then attack red blood cells, multiply and burst them. The bursting cells release a toxin called haemozoin, which causes the recurring chills and high fever.
Some parasites form gametocytes in the blood.
A female Anopheles sucks the blood, taking in gametocytes. Fertilisation and development take place in the mosquito's gut, forming sporozoites that move to its salivary glands, ready for the next bite.
Prevention: control mosquito breeding by removing stagnant water, spraying insecticides, using mosquito nets and screens, and introducing fish such as Gambusia that eat mosquito larvae.
Immunity
The body's ability to fight disease-causing organisms is immunity.
Innate immunity is present from birth and is non-specific. It has four kinds of barrier:
Barrier | Examples |
|---|---|
Physical | skin; mucus lining of the respiratory, digestive and urogenital tracts |
Physiological | stomach acid, saliva, tears |
Cellular | WBCs such as neutrophils and monocytes, natural killer cells, and macrophages that engulf microbes |
Cytokine | interferons, released by virus-infected cells, protect neighbouring cells |
Acquired immunity is pathogen-specific and has memory. The first encounter gives a slow, weak primary response; a later encounter with the same pathogen gives a fast, strong secondary (anamnestic) response.
It is carried out by two kinds of lymphocytes:
B-lymphocytes produce antibodies into the blood: humoral immunity.
T-lymphocytes help B cells and directly attack infected cells: cell-mediated immunity. This is why organ transplants can be rejected; tissue and blood matching and immunosuppressants are needed.
Antibodies are Y-shaped proteins made of four peptide chains: two light and two heavy chains (H₂L₂). Types include IgA, IgM, IgE and IgG.
Active and passive immunity:
Active | Passive | |
|---|---|---|
How | the body makes its own antibodies after exposure to an antigen (infection or vaccine) | ready-made antibodies are given |
Speed | slow to develop | immediate |
Duration | long-lasting | short-lived |
Examples | vaccination; recovering from measles | colostrum (IgA) in a mother's first milk; antibodies passing across the placenta; anti-tetanus and anti-venom injections |
Vaccination and immunisation
A vaccine contains inactivated or weakened pathogens, or their proteins. It triggers a primary response and the formation of memory B and T cells, so that a real infection later meets a fast secondary response. Vaccines can also be made by recombinant DNA technology, as for hepatitis B (produced in yeast).
Allergies and autoimmunity
An allergy is an exaggerated immune response to harmless substances (allergens) such as dust mites, pollen and animal dander. Allergies involve IgE antibodies and chemicals such as histamine and serotonin from mast cells. They are treated with antihistamines, adrenaline and steroids.
In autoimmune diseases, the immune system attacks the body's own cells. Rheumatoid arthritis is an example.
The immune system's organs
Primary lymphoid organs: the bone marrow (where lymphocytes are made and B cells mature) and the thymus (where T cells mature).
Secondary lymphoid organs: the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine, and the appendix, where lymphocytes meet antigens. MALT (mucosa-associated lymphoid tissue) makes up about half the body's lymphoid tissue.
AIDS
AIDS (Acquired Immuno Deficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus), a retrovirus with RNA as its genetic material.
It spreads through sexual contact with an infected person, transfusion of contaminated blood, shared needles, and from an infected mother to her child through the placenta. It does not spread by touching, hugging or sharing food.
HIV enters macrophages, where its RNA is copied into DNA by the enzyme reverse transcriptase. The viral DNA joins the host's DNA, and the cells produce new viruses. The virus then attacks helper T-lymphocytes (T_H cells), steadily reducing their number. The person becomes vulnerable to infections such as Mycobacterium, viruses, fungi and Toxoplasma. There may be a long gap (from a few months to many years) between infection and symptoms.
Diagnosis uses the ELISA test. Antiretroviral drugs can prolong life but not cure AIDS. Prevention: safe blood, disposable needles and syringes, safe sex, and awareness. NACO and many NGOs in India work on education.
Cancer
In normal cells, growth and division are tightly controlled. Normal cells show contact inhibition: contact with other cells stops further growth. Cancer cells lose this property and keep dividing, forming tumours.
Benign tumours stay in one place and cause little damage.
Malignant tumours grow rapidly, invade surrounding tissues, and spread through the blood to other sites to form new tumours. This spreading is metastasis, the most feared property of cancer.
Causes (carcinogens): ionising radiation (X-rays, gamma rays), non-ionising UV radiation, chemicals in tobacco smoke, and oncogenic viruses. Normal genes called proto-oncogenes can become cancer-causing oncogenes when activated.
Detection: biopsy and histopathology, blood and bone marrow tests (for leukaemia), radiography, CT scans and MRI, and antibodies against cancer-specific antigens.
Treatment: surgery, radiation therapy, chemotherapy, and immunotherapy using biological response modifiers such as α-interferon, which activate the immune system.
Drugs and alcohol abuse
Drug | Source | Effect |
|---|---|---|
Opioids (heroin, also called smack) | from morphine, extracted from the latex of the poppy Papaver somniferum | depressant: slows body functions |
Cannabinoids (marijuana, hashish, charas, ganja) | flowers, leaves and resin of Cannabis sativa | affect the cardiovascular system |
Cocaine (coke, crack) | coca plant Erythroxylum coca | stimulant; produces euphoria; excess causes hallucinations |
Some medicines, such as barbiturates, amphetamines and benzodiazepines, are useful when prescribed but are also misused. Tobacco contains nicotine, which raises blood pressure and heart rate; smoking is linked to cancers of the lung, throat and mouth, bronchitis and emphysema. Carbon monoxide in smoke reduces the oxygen carried by haemoglobin.
Addiction and dependence: repeated use creates a psychological attachment (addiction) and physical dependence; stopping suddenly causes withdrawal symptoms. Alcohol abuse damages the liver (cirrhosis) and nervous system.
Prevention: avoid undue peer pressure, seek help from parents, teachers and professionals, watch for danger signs (falling grades, isolation, change in friends), and get counselling and medical help.
Summary
Pathogens include bacteria, viruses, protozoa, fungi and worms; malaria needs humans and female Anopheles.
Innate immunity is non-specific; acquired immunity is specific with memory (B cells for antibodies, T cells for cell-mediated immunity).
Antibodies have four chains (H₂L₂); vaccines give active immunity, colostrum gives passive immunity.
HIV destroys helper T cells; diagnosis by ELISA.
Malignant tumours metastasise; carcinogens include radiation, chemicals and viruses.