Human Health and Disease

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 7.

Why do you catch a cold again and again, but chickenpox only once?

Your body is under constant attack, and it keeps a diary. Some invaders are met for the first time and cause trouble. Others have been met before, and your body recognises them within hours and ends the fight before you feel anything. This chapter is about that fight: the pathogens that cause disease, the defence system that stops them, the two big diseases where defence fails (AIDS and cancer), and the drugs that damage the body from the inside.

What is health?

Health is a state of complete physical, mental and social well-being, not merely the absence of disease. Three things decide it: your genes (genetic disorders), infections, and your lifestyle (food, water, rest, exercise and habits).

Diseases fall into two groups. Infectious diseases are caused by a pathogen (a disease-causing organism) and can pass from one person to another. Non-infectious diseases, such as cancer, are not passed on this way. Cancer is the major non-infectious killer, while AIDS is the fatal infectious disease the chapter singles out.

Common diseases of humans

CUET asks this part mostly as match-the-column. Learn each row as a set: pathogen, how it spreads, and the one clue that identifies it.

Why typhoid is hard to stop. Some people recover but keep the bacteria in their body and go on shedding them. They are carriers. The most famous was Mary Mallon, a cook nicknamed "Typhoid Mary", who infected many people through the food she prepared without ever being ill herself. In severe typhoid the intestine can even be perforated, which can be fatal.

Malaria, step by step. The parasite needs two hosts, a human and the female Anopheles mosquito, and the mosquito is the vector (the carrier that transmits the pathogen).

The life cycle in one line: infected mosquito injects sporozoites → they multiply in liver cells → they attack red blood cells (RBCs) and multiply → RBCs burst and release haemozoin (a toxin), causing the chill and fever cycle → some parasites become gametocytes → a mosquito sucks them up → fertilisation and development in the mosquito's gut → sporozoites reach the salivary glands → ready for the next bite.

Plasmodium falciparum causes malignant malaria, the most serious form, and it can be fatal. The other two species to know are P. vivax and P. malariae.

Beyond the textbook. For the exam: haemozoin is the toxin that causes the periodic chill and fever of malaria. In reality: haemozoin is the pigment the parasite leaves behind when it digests the haemoglobin of your RBCs. The fever is largely your own immune system reacting to the material released each time a batch of RBCs bursts, which is why it comes in waves. Two more species, P. ovale and P. knowlesi, can infect humans, but the exam tests only the three above. Why it matters: knowing the fever is a defence reaction explains why the cycle repeats every few days, not constantly.

Pause and predict: why does malaria spread so well in a village with stagnant ponds, even if nobody in the village ever meets an infected person?

Check your answer: because malaria is not passed from person to person. It needs the mosquito, and stagnant water is where mosquitoes breed. Remove the breeding places and you break the cycle.

Beyond the textbook. For the exam: filariasis is spread by "female mosquito vectors", and dengue and chikungunya are spread by Aedes mosquitoes. In reality: the usual vector of filariasis in India is the Culex mosquito, which breeds in dirty, polluted water, while Aedes breeds in clean water collected in coolers, tyres and pots. Why it matters: the three diseases, three mosquitoes pairing (malaria with Anopheles, filariasis with Culex, dengue with Aedes) is a common source of wrong answers.

How diseases are prevented. Prevention works at three levels. Personal hygiene means keeping the body clean and using safe drinking water, food and vegetables. Public hygiene means proper disposal of waste and excreta, periodic cleaning of water reservoirs, and standard practices for potable water supply. Vector control means avoiding stagnant water, cleaning household coolers regularly, using mosquito nets, putting Gambusia fish (which eat mosquito larvae) in ponds, and spraying insecticides in drains and ditches. For vector-borne diseases like malaria and filariasis, controlling the vector matters most. Vaccines and antibiotics add to this, and they work through the immune system, which we turn to next.

Immunity: your defence system

Immunity is the ability of the body to fight disease-causing organisms. It comes in two kinds: innate (present from birth) and acquired (built up after meeting a pathogen).

Innate immunity has four kinds of barriers. Think of a castle: a wall, a moat, guards inside, and an alarm system.

Acquired immunity: the body keeps a diary. The first time a pathogen enters, the response is slow and weak: the primary response. If the same pathogen comes back, the response is much faster and stronger: the secondary (anamnestic) response. That is memory, and it is why you rarely suffer the same infection twice.

Two kinds of lymphocytes do the work. B lymphocytes produce proteins called antibodies into the blood to fight pathogens: this is humoral immunity (the old word "humor" means body fluid). T lymphocytes do not make antibodies. They help B cells to make them and they mediate cell-mediated immunity (CMI), in which the T cells themselves attack infected or foreign cells.

Antibody structure. An antibody has four polypeptide chains: two identical heavy chains and two identical light chains, so it is written H2L2. The classes to know are IgA, IgM, IgE and IgG. (Immunoglobulin classes are named Ig plus a letter; IgD also exists.)

Self and non-self. CMI is why a transplanted organ is rejected. The body can tell its own cells (self) from foreign cells (non-self), so a graft from another person is attacked. Doctors need good tissue matching and give immunosuppressive drugs to stop this.

Pause and predict: why does the second dose of a vaccine, or a booster, give stronger protection than the first?

Check your answer: the first dose produces only a weak primary response, but it leaves behind memory B and T cells. The booster triggers the secondary response from those memory cells, which is faster and much more intense.

Active and passive immunity.

Vaccination in one idea. A vaccine contains a weakened or inactivated pathogen, or just some of its proteins. It cannot make you ill, but your B and T cells treat it as the real thing and form memory cells. When the real pathogen arrives, the secondary response wipes it out. Tetanus antitoxin and snake antivenom work differently: they are preformed antibodies, used when there is no time to wait for your own response. Using recombinant DNA technology, yeast has been made to produce the antigenic protein of the hepatitis B virus, giving the hepatitis B vaccine.

Pause and predict: a person is bitten by a venomous snake and the doctor injects antivenom. Is this active or passive immunity? Will the person be protected from the next bite?

Check your answer: passive, because ready-made antibodies were given. There is no memory, so the next bite would need the same treatment again.

When immunity overreacts. In allergy, the immune system reacts too strongly to harmless substances in the environment, called allergens (dust mites, pollen, animal dander). The antibodies involved are IgE. Mast cells release histamine and serotonin, causing sneezing, watery eyes, a runny nose and breathing difficulty. Drugs such as anti-histamines, adrenaline and steroids quickly reduce the symptoms. More children in cities now suffer from allergies and asthma, linked to a modern lifestyle with lower immunity and more sensitivity to allergens.

In auto-immune disease, the body fails to tell self from non-self and attacks its own cells. Rheumatoid arthritis is the standard example.

Lymphoid organs. These are the organs where lymphocytes are made, mature and meet antigens.

The bone marrow is the main lymphoid organ where all blood cells, including lymphocytes, are produced. The thymus is a lobed organ near the heart, under the breastbone. It is large at birth but shrinks with age. The spleen is bean-shaped, full of lymphocytes and phagocytes, and filters the blood by trapping blood-borne microbes. MALT (mucosa-associated lymphoid tissue) lies in the lining of the respiratory, digestive and urogenital tracts and makes up about 50 per cent of the lymphoid tissue in the body.

AIDS: when the defence system is the target

AIDS (Acquired Immuno Deficiency Syndrome) was first reported in 1981. It is caused by HIV (Human Immunodeficiency Virus), a retrovirus: an enveloped virus whose genome is RNA. The name tells the story. Acquired means you get it, not inherit it. Immuno deficiency means the immune system is weakened. Syndrome means a group of symptoms.

How it spreads. Only by close contact between body fluids: sexual contact with an infected person, transfusion of contaminated blood and blood products, sharing infected needles, and from an infected mother to her child through the placenta. It does not spread by touch, shaking hands or sharing food. People at higher risk include those with multiple sexual partners, intravenous drug users and people who need repeated blood transfusions.

What the virus does inside you. HIV enters the body, first infects macrophages, and then helper T lymphocytes (TH cells). Inside the cell, reverse transcriptase makes a DNA copy of the viral RNA. That DNA is stitched into the host cell's own DNA, and the cell is then forced to make new virus particles. The infected macrophage acts like an HIV factory, producing virus without being destroyed, while the TH cells are used up as new viruses burst out and infect more TH cells.

Why losing TH cells is catastrophic. Helper T cells are the managers: B cells need them to make antibodies and T cells need them to launch CMI. As their number falls, the patient cannot fight even mild infections and falls prey to bacteria such as Mycobacterium, viruses, fungi and parasites like Toxoplasma. Common signs are persistent fever, diarrhoea and weight loss.

Pause and predict: a person is infected with HIV today but looks perfectly healthy for years. Does this mean the virus is inactive?

Check your answer: no. The time between infection and symptoms ranges from a few months to 5 to 10 years, during which the virus is steadily destroying TH cells and the person can still pass it on.

Diagnosis, treatment and prevention. The widely used test is ELISA (Enzyme Linked Immuno-Sorbent Assay). Anti-retroviral drugs can prolong life but are not a complete cure. Prevention is the real weapon: NACO (National AIDS Control Organisation) and other agencies run awareness programmes, ensure safe blood, promote disposable needles, distribute condoms free of cost and encourage safe sex and regular check-ups.

Beyond the textbook. For the exam: ELISA is the diagnostic test for AIDS and the HIV genome is RNA. In reality: ELISA detects the antibodies your body makes against HIV (or sometimes viral proteins), and antibodies take some weeks to appear, so a very recent infection can be missed. Also, being HIV positive and having AIDS are not the same thing: HIV is the virus, AIDS is the late stage when the immune system has collapsed. Why it matters: a statement that treats "HIV" and "AIDS" as one word is often the wrong option in a statement-based question.

Cancer: cells that forget to stop

Normal cells stop dividing when they touch their neighbours. This is contact inhibition. Cancer cells have lost it, so they keep dividing and pile up into a lump called a tumour.

What causes cancer. Agents that cause cancer are carcinogens, and they work by damaging DNA or switching on growth genes.

Normal cells carry genes called cellular oncogenes (c-onc) or proto-oncogenes. When these are activated by a carcinogen, a normal cell is transformed into a cancer cell.

Beyond the textbook. For the exam: proto-oncogenes are normal genes that cause cancer when activated. In reality: proto-oncogenes are not villains waiting to strike. They are the normal genes that tell a cell when to grow and divide. Cancer starts when they get stuck "on" through mutation, or when the opposite kind of gene, the tumour suppressor genes that put the brakes on division, are lost. Why it matters: it explains why almost any cause of DNA damage, from tobacco to UV, can lead to cancer.

Detection. Biopsy (a thin slice of the suspected tissue is stained and examined under a microscope, called histopathology), blood and bone marrow tests (for increased cell counts in leukaemia), radiography (X-rays), CT scan (uses X-rays to build a three dimensional image of an organ), MRI (uses strong magnetic fields and non-ionising radiation to detect changes), antibodies against cancer-specific antigens, and molecular biology techniques to find genes that make a person susceptible to certain cancers.

Treatment. Surgery removes the tumour. Radiotherapy lethally irradiates tumour cells while sparing the surrounding tissue as far as possible. Chemotherapy uses drugs, many of which have side effects like hair loss and anaemia. Immunotherapy gives biological response modifiers such as α-interferon that activate the immune system. This is needed because tumour cells often avoid detection and destruction by the immune system. Most cancers are treated with a combination of these.

Drugs and alcohol: when chemistry hijacks the brain

Drugs that affect the brain bind to receptors that normally respond to the body's own chemical messengers. That is why they are so powerful, and why different plants give very different effects.

Hallucinogenic plants to know are Atropa belladonna and Datura. Barbiturates, amphetamines and benzodiazepines are medicines for depression and insomnia that are also abused.

What tobacco does. Smoking is linked to cancers of the lung, urinary bladder and throat, bronchitis, emphysema, coronary heart disease and gastric ulcer. Chewing tobacco causes oral cancer. Smoking also raises carbon monoxide in the blood, which reduces the amount of haem-bound oxygen and causes oxygen deficiency in the body.

Beyond the textbook. For the exam: nicotine is a stimulant that releases adrenaline and noradrenaline, and morphine is the sedative and painkiller. In reality: the same plant can give both a medicine and a drug of abuse. Morphine is a vital painkiller in hospitals, while heroin, its chemical modification, crosses into the brain faster and is far more addictive. Why it matters: the examiner's favourite trick is to give nicotine the properties of morphine.

Adolescence and addiction. Adolescence is the bridge between childhood and adulthood. Curiosity, the need for adventure, stress, peer pressure and the influence of media and family can push teenagers to try drugs and alcohol. Addiction is a psychological attachment to the euphoria and temporary well-being a drug gives. With repeated use, the body develops tolerance (you need more for the same effect) and dependence. Dependence means the body needs a regular supply, and stopping triggers a withdrawal syndrome: anxiety, shakiness, nausea and sweating, which can be severe enough to need medical supervision.

What abuse does. Immediate effects include reckless behaviour, vandalism and violence. An overdose can cause coma and death from respiratory failure, heart failure or cerebral haemorrhage. Injecting drugs with shared needles spreads AIDS and hepatitis B. Long-term alcohol abuse damages the nervous system and causes cirrhosis of the liver. Sportspersons who misuse anabolic steroids pay for it too. In females these cause masculinisation (male-pattern hair growth, deepening of voice, enlarged clitoris, abnormal menstrual cycles) and in males acne, mood swings, smaller testes, reduced sperm production and kidney and liver problems.

Prevention and cure. Avoid undue peer pressure, seek education and counselling, ask parents and peers for help, watch for the danger signs of abuse, and seek professional and medical help from psychologists, psychiatrists and de-addiction programmes.

Exam traps

60-second revision

Where this chapter connects

• Earlier in Class XII: Reproductive Health covers sexually transmitted infections, including HIV, and how they are prevented.
• Same class: Microbes in Human Welfare shows the helpful side of microbes, and cyclosporin A, an immunosuppressant used after organ transplants.
• Biotechnology: Biotechnology and its Applications explains how recombinant DNA technology produces vaccines and how gene therapy treats inherited disorders.
• Class XI link: Body Fluids and Circulation introduces the blood cells, including the leucocytes that carry out cellular immunity.