Human Health and Disease: common doubts, answered
The questions students ask most often about Human Health and Disease, each with a short answer. For the full chapter, read the Human Health and Disease notes.
Health and disease
Read this section in the notes →What is the definition of health?
Health is a state of complete physical, mental and social well-being, not merely the absence of disease. A person free of illness but under constant stress or socially isolated is therefore not fully healthy. Balanced diet, hygiene, regular exercise and awareness help maintain it.
Bacterial and viral diseases
Read this section in the notes →How is typhoid diagnosed and what causes it?
Typhoid is caused by the bacterium Salmonella typhi and is confirmed by the Widal test. It spreads through contaminated food and water, and gives sustained high fever of 39 to 40 °C along with weakness and stomach pain. Mary Mallon, known as Typhoid Mary, was a famous carrier.
What is the difference between pneumonia and the common cold?
Pneumonia is caused by bacteria such as Streptococcus pneumoniae and Haemophilus influenzae, and infects the alveoli of the lungs, which fill with fluid. The common cold is caused by rhino viruses and infects the nose and respiratory passage but not the lungs. So pneumonia is the more serious lung infection.
Malaria and amoebiasis
Read this section in the notes →Which stage of Plasmodium enters the human body and what causes malarial fever?
Sporozoites enter the body through the bite of an infected female Anopheles mosquito. The fever with chill appears when infected red blood cells burst and release a toxin, haemozoin, which recurs every 3 to 4 days. Plasmodium falciparum causes the most serious form, called malignant malaria.
How does amoebiasis spread?
Amoebiasis is caused by Entamoeba histolytica, which lives in the large intestine and causes constipation, abdominal pain and cramps, and stools with excess mucus and blood clots. Its cysts spread through contaminated food and water, and houseflies act as mechanical carriers, carrying the parasite on their body.
Worms, fungi and prevention
Read this section in the notes →What is filariasis and which parasites cause it?
Filariasis, also called elephantiasis, is caused by the filarial worms Wuchereria bancrofti and W. malayi. The worms live in the lymphatic vessels, mostly of the lower limbs, and cause chronic inflammation and swelling of the organs. It is transmitted by the bite of female mosquitoes.
How can mosquito-borne diseases be prevented?
By stopping mosquitoes from breeding and biting. Avoid stagnant water in coolers and tyres, use mosquito nets and repellents, spray insecticides, and introduce larva-eating fish such as Gambusia in ponds. Aedes mosquitoes spread dengue and chikungunya, while Anopheles carries malaria, so the control of breeding sites matters for all.
Innate immunity
Read this section in the notes →What are the four types of barriers in innate immunity?
They are physical, physiological, cellular and cytokine barriers. Skin and mucus are physical, acid in the stomach and saliva are physiological, phagocytic cells like neutrophils and macrophages are cellular, and interferons from virus-infected cells are cytokine barriers. Innate immunity is non-specific and is present from birth.
Acquired immunity
Read this section in the notes →What is the difference between innate and acquired immunity?
Innate immunity is non-specific and present at birth, whereas acquired immunity is specific, develops after exposure to a pathogen and is characterised by memory. The memory makes the second response of the body much faster and stronger than the first. B cells and T cells are its main carriers.
What is the difference between B cells and T cells?
B cells make antibodies that circulate in the blood, giving humoral immunity. T cells do not make antibodies, but help B cells to make them and mediate cell-mediated immunity, which is why they are responsible for graft rejection. B cells mature in the bone marrow and T cells in the thymus.
What is the structure of an antibody?
An antibody is a Y-shaped protein made of four polypeptide chains: two light and two heavy, written H₂L₂. The tips bind the antigen. The main types are IgA, IgM, IgE and IgG. They are produced against specific antigens and neutralise the pathogen.
Active and passive immunity
Read this section in the notes →What is the difference between active and passive immunity?
In active immunity the body makes its own antibodies after exposure to a pathogen or a vaccine, so it is slow but long-lasting. In passive immunity ready-made antibodies are given from outside, so it is quick but short-lived. Colostrum, tetanus antitoxin and snake antivenom are examples of passive immunity.
How does vaccination work?
A vaccine contains a weakened or inactivated pathogen, or its antigen, which stimulates the body to make antibodies and memory cells without causing disease. When the real pathogen enters later, memory cells give a quick and strong response. The hepatitis B vaccine is made in yeast using recombinant DNA technology.
Allergy and auto-immunity
Read this section in the notes →What causes allergy?
Allergy is the exaggerated response of the immune system to certain antigens present in the environment, called allergens, such as pollen and dust. IgE antibodies are produced, and the mast cells release histamine and serotonin, causing sneezing, watery eyes and breathing difficulty. Anti-histamine, adrenaline and steroids reduce symptoms.
Lymphoid organs
Read this section in the notes →What are primary and secondary lymphoid organs?
Primary lymphoid organs are where lymphocytes mature and become antigen-sensitive, and they are the bone marrow and thymus. Secondary lymphoid organs, the spleen, lymph nodes, tonsils, Peyer's patches and appendix, are where lymphocytes interact with the antigen and multiply. Infection is fought mainly in secondary organs.
AIDS
Read this section in the notes →How does HIV cause AIDS?
HIV is a retrovirus. After entering the body it infects macrophages, where it makes more virus using reverse transcriptase, and then helper T cells, whose number falls. As a result the immunity declines, and the person suffers repeated infections. There is usually a time-lag of 5 to 10 years between infection and the development of AIDS.
How is AIDS diagnosed and prevented?
The common screening test is ELISA, based on antigen-antibody interaction, and PCR is also used. Prevention depends on avoiding the routes of transmission: use disposable needles, screened blood, safe sex with condoms, and care in pregnancy. AIDS does not spread through casual contact such as handshakes or sharing food.
Cancer
Read this section in the notes →What is the difference between benign and malignant tumours?
A benign tumour stays confined to its site and does not spread, while a malignant tumour spreads to other parts by metastasis. Cancer cells lose contact inhibition, which normally stops cells from dividing when they touch their neighbours. Metastasis is what makes malignant tumours dangerous.
Drugs, alcohol and addiction
Read this section in the notes →What is the difference between heroin and cocaine?
Heroin, or smack, is diacetylmorphine, made from morphine of the poppy Papaver somniferum, and it is a depressant that slows body functions. Cocaine comes from Erythroxylum coca and is a stimulant: it interferes with the transport of the neurotransmitter dopamine, producing a sense of euphoria.
How does smoking harm the body?
Tobacco smoke contains nicotine, which raises the level of adrenaline and noradrenaline and thus the blood pressure and heart rate. Carbon monoxide in the smoke binds haemoglobin and reduces the oxygen it carries. Smoking is linked to cancers of the lung and throat, bronchitis, emphysema and coronary heart disease.
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