Human Health and Disease: NEET notes

1. Health and disease

NCERT §7 (chapter introduction)

• Early Greeks such as Hippocrates, and the Indian Ayurveda system, pictured health as a balance of body 'humors'; a person with excess 'black bile' was supposed to be hot-tempered and feverish. This idea came from reflection alone, not from experiment.
• The humor idea fell when William Harvey showed blood circulation by experiment, and when a thermometer found normal body temperature in so-called black-bile people.
• Later biology showed that the mind acts on the immune system through the neural and endocrine systems, and the immune system keeps us healthy, so mental state can affect health.
• Health is affected by genetic disorders (defects present at birth or inherited from parents), by infections, and by life style: food and water, rest and exercise, and habits.
• Health is not just the absence of disease or plain physical fitness; it means being completely well in body, in mind and in social life.
• Healthy people work more efficiently, which raises productivity and prosperity; health also lengthens life and lowers infant and maternal mortality.
• Good health rests on a balanced diet, personal hygiene, regular exercise and yoga, together with awareness of diseases, vaccination, proper waste disposal, vector control and clean food and water.
• A disease is present when one or more organs or systems work badly and signs and symptoms appear. Infectious diseases pass easily from person to person, and some, like AIDS, kill; of the non-infectious diseases, cancer causes the most deaths.

2. Bacterial and viral diseases

NCERT §7.1

• Pathogens are disease-causing organisms: bacteria, viruses, fungi, protozoans, helminths and others. Most parasites are pathogens, because they harm the host they live in or on.
• A pathogen must adapt to its host. One that enters through the gut has to survive the low pH of the stomach and resist the digestive enzymes.
• Typhoid is caused by the bacterium Salmonella typhi. It enters the small intestine in contaminated food and water and spreads to other organs through the blood.
• Typhoid signs: a high fever that stays at 39° to 40°C, with weakness, pain in the stomach, constipation, headache and poor appetite; severe cases can end in intestinal perforation and death. The Widal test confirms it.
• Mary Mallon ('Typhoid Mary') was a cook and a typhoid carrier who kept spreading the disease for years through the food she prepared.
• Pneumonia is caused by Streptococcus pneumoniae and Haemophilus influenzae. It infects the alveoli, which fill with fluid and make breathing very hard. The patient has fever, chills, cough and headache, and when it is severe, the lips and finger nails go grey to bluish.
• Pneumonia spreads by inhaling droplets or aerosols from an infected person, or by sharing their glasses and utensils. Dysentery, plague and diphtheria are other bacterial diseases.
• The common cold is caused by rhino viruses, which attack the nose and the respiratory passage but spare the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough, headache and tiredness usually last 3-7 days.
• Cold droplets from coughs and sneezes are inhaled directly or picked up from contaminated objects such as pens, books, cups, doorknobs and a computer keyboard or mouse.

3. Malaria and amoebiasis

NCERT §7.1

• Malaria is caused by the protozoan Plasmodium. Different species (P. vivax, P. malariae and P. falciparum) cause different types; malignant malaria, caused by P. falciparum, is the most serious and can be fatal.
• Plasmodium enters a person as sporozoites, the infectious form, through the bite of an infected female Anopheles mosquito.
• The parasites first multiply inside liver cells, then attack red blood cells and make them burst. Each burst releases a toxic substance, haemozoin, which causes the chill and high fever that return every three to four days.
• In the human, the parasites reproduce asexually in liver cells and in RBCs; some become gametocytes, the sexual stages, inside RBCs.
• A female Anopheles biting an infected person takes up these stages. Fertilisation and further development happen in the mosquito, and the new sporozoites are stored in its salivary glands, ready for the next bite.
• The malarial parasite needs two hosts, human and mosquito, to complete its life cycle; the female Anopheles is also its vector (transmitting agent).
• Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, a protozoan in the large intestine. Symptoms: constipation, abdominal pain and cramps, and stools with excess mucous and blood clots.
• Houseflies act as mechanical carriers, moving the parasite from an infected person's faeces to food. Drinking water and food contaminated with faecal matter are the main source of infection.

4. Worms, fungi and prevention

NCERT §7.1

• Ascaris, the common round worm, is an intestinal parasite that causes ascariasis: internal bleeding, muscular pain, fever, anaemia and blockage of the intestinal passage.
• Ascaris eggs leave in the faeces of an infected person and contaminate soil, water and plants; a healthy person is infected through contaminated water, vegetables and fruits.
• Wuchereria (W. bancrofti and W. malayi), the filarial worms, live for many years usually in the lymphatic vessels of the lower limbs and cause a slow, chronic inflammation called elephantiasis or filariasis. Genital organs are often affected too, with gross deformities. Female mosquito vectors carry the worms by their bite.
• Ringworm, one of the most common infectious diseases, is caused by fungi of the genera Microsporum, Trichophyton and Epidermophyton. It shows as dry, scaly lesions on the skin, nails and scalp, with intense itching.
• Heat and moisture favour these fungi, so they thrive in skin folds such as the groin and between the toes. Ringworm is picked up from soil or from an infected person's towels, clothes or comb.
• Personal hygiene (a clean body, clean drinking water, food, vegetables and fruit) and public hygiene (disposal of waste and excreta, periodic cleaning and disinfection of reservoirs, pools, cesspools and tanks, and hygienic public catering) matter most for food- and water-borne diseases: typhoid, amoebiasis and ascariasis.
• For air-borne diseases such as pneumonia and the common cold, one must also avoid close contact with infected persons and their belongings.
• For vector-borne malaria and filariasis, the key step is to control the vectors and their breeding places: no stagnant water near homes, cleaning household coolers, mosquito nets, Gambusia fish in ponds to eat mosquito larvae, insecticide in ditches, drains and swamps, and wire mesh on doors and windows. Aedes mosquitoes spread dengue and chikungunya.
• Vaccination eradicated smallpox, and has largely controlled polio, diphtheria, pneumonia and tetanus. Antibiotics and other drugs treat many infectious diseases.

5. Innate immunity

NCERT §7.2, §7.2.1

• We meet infectious agents every day, yet only a few exposures cause disease, because the body defends itself. The host's overall ability, given by the immune system, to fight disease-causing organisms is immunity.
• Immunity is of two types: innate and acquired.
• Innate immunity is non-specific and present at birth. It works through four types of barriers to foreign agents.
• Physical barriers: the skin is the main one. Mucus coating the epithelium of the respiratory, gastrointestinal and urogenital tracts traps entering microbes.
• Physiological barriers: stomach acid, saliva in the mouth and tears from the eyes all stop microbes from growing.
• Cellular barriers: in blood, polymorpho-nuclear leukocytes (PMNL-neutrophils), monocytes and natural killer cells (a type of lymphocyte), and in tissues, macrophages, can phagocytose and destroy microbes.
• Cytokine barriers: a cell infected by a virus releases proteins called interferons, which shield the cells around it that are not yet infected.

6. Acquired immunity

NCERT §7.2.2

• Acquired immunity is pathogen-specific and has memory.
• The first encounter with a pathogen gives a primary response of low intensity. A later encounter with the same pathogen gives a much stronger secondary, or anamnestic, response, as if the body remembers the first meeting.
• Both responses are carried out by two kinds of lymphocytes in the blood: B-lymphocytes and T-lymphocytes.
• B-lymphocytes release proteins called antibodies into the blood to fight pathogens. T-cells do not secrete antibodies themselves but help B cells produce them.
• An antibody has four peptide chains, two small light chains and two longer heavy chains, so it is written H2L2. IgA, IgM, IgE and IgG are some of the antibodies our body makes.
• Because antibodies are found in the blood, the antibody-mediated response is called the humoral immune response.
• The second type of acquired response is cell-mediated immunity (CMI), carried out by T-lymphocytes.
• Organs such as heart, eye, liver or kidney can be transplanted only after tissue matching and blood group matching, and the patient still takes immuno-suppressants for life. The body tells 'self' from 'non-self', and CMI is responsible for graft rejection.

7. Active and passive immunity

NCERT §7.2.3, §7.2.4

• Active immunity: the host is exposed to antigens (living or dead microbes or other proteins) and makes its own antibodies. It is slow and takes time to reach full effect.
• Active immunity follows either deliberate immunisation or natural infection.
• Passive immunity: ready-made antibodies are given directly to protect the body.
• Colostrum, the yellowish fluid a mother secretes in the first days of lactation, is rich in IgA antibodies that protect the infant; the foetus also receives antibodies across the placenta. Both are passive immunity.
• Vaccination rests on immune memory. A vaccine (antigenic proteins of a pathogen, or the pathogen inactivated or weakened) makes the body produce antibodies that neutralise the real pathogen later, and memory B and T cells that recognise it quickly and flood it with antibodies.
• When a quick response is needed against a deadly microbe, as in tetanus, preformed antibodies or an antitoxin (antibodies against the toxin) are injected. The snakebite injection likewise carries preformed antibodies against the venom. This is passive immunisation.
• Recombinant DNA technology makes a pathogen's antigenic polypeptides in bacteria or yeast, allowing large-scale production of vaccines, for example the hepatitis B vaccine made from yeast.

8. Allergy and auto-immunity

NCERT §7.2.5, §7.2.6

• Allergy is an exaggerated response of the immune system to certain antigens in the environment. The substances that trigger it are allergens, and the antibodies made against them are of the IgE type.
• Common allergens: mites in dust, pollen and animal dander. They differ from place to place, which is why symptoms can start in a new place and vanish on leaving it.
• Symptoms are sneezing, watery eyes, running nose and difficulty in breathing. They come from chemicals such as histamine and serotonin released by mast cells.
• To find the cause, doctors expose the patient to tiny doses of suspected allergens, or inject them, and watch which one produces a reaction. Anti-histamines, adrenalin and steroids quickly reduce the symptoms.
• Modern life style seems to have lowered immunity and raised sensitivity: more children in Indian metro cities suffer from allergies and asthma, possibly because of the protected environment they grow up in.
• Memory-based acquired immunity evolved in higher vertebrates from the ability to tell foreign organisms from self cells; they can distinguish foreign molecules as well as foreign organisms.
• Sometimes, for genetic and other unknown reasons, the body attacks its own cells. The damage that follows is an auto-immune disease; rheumatoid arthritis is an example.

9. Lymphoid organs

NCERT §7.2.7

• The immune system is made of lymphoid organs, tissues, cells and soluble molecules such as antibodies. It recognises foreign antigens, responds to them and remembers them, and it is involved in allergy, auto-immune disease and organ transplantation.
• Lymphoid organs are where lymphocytes originate and/or mature and proliferate.
• Primary lymphoid organs are the bone marrow and thymus, where immature lymphocytes become antigen-sensitive lymphocytes.
• Secondary lymphoid organs are the spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and the appendix. Mature lymphocytes migrate here, meet antigens and proliferate into effector cells.
• Bone marrow is the main lymphoid organ: all blood cells, lymphocytes included, are made there. Bone marrow and thymus both provide micro-environments for T-lymphocytes to develop and mature.
• The thymus is a lobed organ near the heart, beneath the breastbone. It is quite large at birth and keeps shrinking with age, becoming very small by puberty.
• The spleen is a large bean-shaped organ holding mainly lymphocytes and phagocytes. It filters blood by trapping blood-borne micro-organisms and also holds a large reservoir of erythrocytes.
• Lymph nodes are small solid structures along the lymphatic system. They trap micro-organisms and antigens that enter lymph and tissue fluid, and the trapped antigens activate the lymphocytes there.
• Mucosa-associated lymphoid tissue (MALT) lines the respiratory, digestive and urogenital tracts and makes up about 50 per cent of the lymphoid tissue in the human body.

10. AIDS

NCERT §7.3

• AIDS is Acquired Immuno Deficiency Syndrome: an immune deficiency acquired during life, so not congenital, and a syndrome, meaning a group of symptoms. It was first reported in 1981 and in about twenty-five years killed more than 25 million people worldwide.
• Its cause is HIV, the Human Immuno deficiency Virus, a retrovirus: an RNA genome inside an envelope.
• HIV spreads by sexual contact with an infected person, by transfusion of contaminated blood and blood products, by sharing infected needles (intravenous drug abusers) and from an infected mother to her child through the placenta.
• HIV is not spread by touch or physical contact, only through body fluids, so infected people should not be isolated from family and society.
• Between infection and the appearance of AIDS symptoms lies a time-lag that may be months or many years long, usually 5-10 years.
• In the body, HIV enters macrophages. Reverse transcriptase copies its RNA genome into viral DNA, which is incorporated into the host cell's DNA and directs the cell to make virus particles; the macrophage becomes an HIV factory.
• HIV also enters helper T-lymphocytes (TH), replicates and releases progeny viruses that attack more helper T cells. Repeated rounds steadily lower the number of helper T cells; meanwhile the person has bouts of fever, diarrhoea and weight loss.
• With too few helper T cells, the person falls ill with infections that would otherwise be overcome: bacteria (especially Mycobacterium), viruses, fungi and parasites such as Toxoplasma.
• ELISA (enzyme linked immuno-sorbent assay) is a widely used diagnostic test. Anti-retroviral drugs are only partially effective: they prolong life but cannot prevent death.
• With no cure available, prevention is what protects people. NACO, NGOs and WHO programmes work for safe blood, disposable needles and syringes only, free condoms, control of drug abuse, safe sex and regular HIV check-ups for susceptible groups; infection should never be hidden.

11. Cancer

NCERT §7.4

• Cancer is a major cause of death worldwide; more than a million Indians suffer from it. In cancer cells, the normal controls on cell growth and differentiation break down.
• Normal cells show contact inhibition: touching other cells stops their uncontrolled growth. Cancer cells seem to lose this property, so they keep dividing into masses called tumors.
• Benign tumors stay in their original location, do not spread and cause little damage. Malignant tumors are masses of rapidly proliferating neoplastic (tumor) cells that invade and damage the surrounding tissue and starve normal cells by competing for nutrients.
• Metastasis, the most feared property of malignant tumors: cells shed from the tumor travel in the blood and start new tumors wherever they lodge.
• Carcinogens, the agents that turn normal cells neoplastic, are physical, chemical or biological. Ionising radiations (X-rays, gamma rays) and non-ionising UV damage DNA; chemicals in tobacco smoke are a major cause of lung cancer.
• Oncogenic viruses carry viral oncogenes. Normal cells carry cellular oncogenes (c-onc), or proto-oncogenes, which can cause oncogenic transformation when activated under certain conditions.
• Early detection lets many cancers be treated successfully. Detection uses biopsy with histopathology (thin stained sections of suspected tissue examined by a pathologist) and, for leukemias, blood and bone marrow tests for raised cell counts.
• Radiography (X-rays), CT (X-rays building a three-dimensional image) and MRI (strong magnetic fields and non-ionising radiations) detect internal cancers. Antibodies against cancer-specific antigens detect certain cancers, and molecular tests find genes for inherited susceptibility, so carriers can avoid their carcinogen (for example tobacco smoke for lung cancer).
• Treatment: surgery, radiation therapy, chemotherapy and immunotherapy; most cancers get a combination of surgery, radiotherapy and chemotherapy. Radiotherapy irradiates tumor cells lethally while sparing nearby normal tissue; most chemotherapy drugs have side effects such as hair loss and anaemia.
• Tumor cells can avoid detection by the immune system, so patients are given biological response modifiers such as α-interferon, which activate the immune system to destroy the tumor.

12. Drugs, alcohol and addiction

NCERT §7.5

• The commonly abused drugs are opioids, cannabinoids and coca alkaloids, mostly from flowering plants and some from fungi. Opioids bind opioid receptors in the central nervous system and gastrointestinal tract.
• Heroin (smack) is diacetylmorphine, a white, odourless, bitter crystalline compound made by acetylating morphine, which comes from the latex of the poppy Papaver somniferum. Usually snorted or injected, heroin is a depressant that slows body functions. Morphine itself is a very effective sedative and painkiller, useful after surgery.
• Cannabinoids act on cannabinoid receptors, mainly in the brain. They come from the inflorescences of Cannabis sativa; its flower tops, leaves and resin give marijuana, hashish, charas and ganja, taken by inhalation or orally, with effects on the cardiovascular system. Some sportspersons abuse them.
• Cocaine (coke, crack), from the coca plant Erythroxylum coca of South America, interferes with the transport of the neurotransmitter dopamine. Usually snorted, it strongly stimulates the CNS, giving euphoria and more energy; excessive doses cause hallucinations. Atropa belladona and Datura are other hallucinogenic plants.
• Medicines for depression and insomnia, such as barbiturates, amphetamines and benzodiazepines, are often abused. Use of any drug for a non-medicinal purpose, or in amounts or frequency that impair physical, physiological or psychological functions, is drug abuse.
• Tobacco, used for more than 400 years, is smoked, chewed or taken as snuff. Its alkaloid nicotine makes the adrenal gland release adrenaline and nor-adrenaline, which raise blood pressure and heart rate. Smoking raises the risk of lung, throat and urinary bladder cancer, of bronchitis and emphysema, of coronary heart disease and of gastric ulcer; chewing raises the risk of oral cancer. Smoking also raises CO in blood and lowers haem-bound oxygen.
• Adolescence, roughly 12-18 years, is a vulnerable bridge between childhood and adulthood. Curiosity, adventure and experimentation, exam stress, the idea that drugs are 'cool' (helped by media), unstable families and peer pressure push the young towards drugs and alcohol.
• Addiction is a psychological attachment to effects such as euphoria. Repeated use raises the tolerance of the body's receptors, so only higher doses work, and even one use can lead to addiction. Dependence is the body's unpleasant withdrawal syndrome (anxiety, shakiness, nausea, sweating) when regular use stops abruptly; it can be life-threatening.
• Effects: reckless behaviour, vandalism and violence; overdose can cause coma and death by respiratory failure, heart failure or cerebral haemorrhage; sharing needles spreads AIDS and hepatitis B; chronic use damages the nervous system and liver (cirrhosis); use in pregnancy harms the foetus. Anabolic steroids misused in sport cause masculinisation in females, testis shrinkage and reduced sperm in males, and in adolescents premature closure of long-bone growth centres, stunting growth.
• Preventing abuse beats treating it: avoid undue peer pressure, educate and counsel, seek help from parents and peers, watch for danger signs, and seek professional and medical help. Parenting that combines high nurturance with consistent discipline lowers the risk of abuse.

Must-know facts

1. Health: complete physical, mental and social well-being, not just the absence of disease.
2. Typhoid: Salmonella typhi, fever 39° to 40°C, confirmed by the Widal test; Mary Mallon was a carrier.
3. Pneumonia: Streptococcus pneumoniae and Haemophilus influenzae, alveoli fill with fluid; lips and nails turn grey to bluish in severe cases.
4. Common cold: rhino viruses, nose and respiratory passage but not lungs, 3-7 days.
5. Malaria: sporozoites enter by the bite of a female Anopheles; haemozoin from bursting RBCs causes chill and fever every 3-4 days; P. falciparum causes malignant malaria.
6. Amoebiasis: Entamoeba histolytica in the large intestine; houseflies are mechanical carriers.
7. Filariasis (elephantiasis): Wuchereria bancrofti and W. malayi in lymphatic vessels of the lower limbs, spread by female mosquitoes.
8. Ringworm fungi: Microsporum, Trichophyton, Epidermophyton.
9. Gambusia fish eat mosquito larvae; Aedes spreads dengue and chikungunya.
10. Innate barriers: physical, physiological, cellular, cytokine (interferons).
11. Antibody: H2L2, two light and two heavy chains; IgA, IgM, IgE, IgG.
12. B cells make antibodies (humoral); T cells mediate CMI and graft rejection.
13. Colostrum carries IgA: passive immunity. Tetanus antitoxin and snake antivenom are passive immunisation.
14. Hepatitis B vaccine is made in yeast by recombinant DNA technology.
15. Allergy: IgE, histamine and serotonin from mast cells; treated with anti-histamine, adrenalin, steroids.
16. Primary lymphoid organs: bone marrow and thymus. Secondary: spleen, lymph nodes, tonsils, Peyer's patches, appendix. MALT is about 50% of lymphoid tissue.
17. HIV: retrovirus; macrophage is the HIV factory; helper T cells fall; ELISA detects; time-lag usually 5-10 years.
18. Cancer cells lose contact inhibition; metastasis defines malignant tumors; α-interferon is a biological response modifier.
19. Heroin: diacetylmorphine from morphine of Papaver somniferum, a depressant. Cocaine: Erythroxylum coca, blocks dopamine transport.
20. Nicotine releases adrenaline and nor-adrenaline; smoking raises CO and lowers haem-bound oxygen.

Common traps

Key terms

Pathogen

An organism that causes disease, such as a bacterium, virus, fungus, protozoan or helminth.

Widal test

The test that confirms typhoid fever.

Sporozoite

The infectious form of Plasmodium injected by an infected female Anopheles.

Haemozoin

Toxic substance released when infected RBCs burst, causing the recurring chill and fever of malaria.

Vector

An organism that carries a pathogen to a new host, such as the female Anopheles for malaria.

Interferon

Protein secreted by virus-infected cells that protects non-infected cells from the virus.

Anamnestic response

The strong secondary immune response to a pathogen met before.

Humoral immune response

The antibody-mediated response, so named because antibodies are in the blood.

Cell-mediated immunity

The T-lymphocyte-mediated response, responsible for graft rejection.

Colostrum

The yellowish first milk, rich in IgA antibodies for the newborn.

Antitoxin

A preparation of antibodies against a toxin, injected for quick protection as in tetanus.

Allergen

An environmental substance that triggers an allergic, IgE-type response.

Auto-immune disease

Damage caused when the body's immune system attacks its own cells, such as rheumatoid arthritis.

MALT

Mucosa-associated lymphoid tissue in the linings of the major tracts, about half of all lymphoid tissue.

Retrovirus

An enveloped RNA virus that copies its genome into DNA using reverse transcriptase.

Contact inhibition

The stop signal normal cells receive from touching neighbours; cancer cells lose it.

Metastasis

Spread of malignant tumor cells through blood to start tumors at distant sites.

Proto-oncogene

A normal cellular gene (c-onc) that can cause cancer when activated under certain conditions.

Addiction

Psychological attachment to the effects of a drug or alcohol.

Dependence

The body's withdrawal syndrome when regular drug or alcohol use is stopped abruptly.