Common Diseases in Humans

Common diseases in humans are caused by pathogens such as bacteria, viruses, protozoans, helminths and fungi, while cancer is the major non-infectious cause of death. This page covers health and the factors that affect it, typhoid, pneumonia, the common cold, malaria and the life cycle of Plasmodium, amoebiasis, ascariasis, filariasis, ringworm, their prevention, and cancer, as in the NCERT Class 12 chapter Human Health and Disease. NEET often asks about disease and pathogen pairs, the malaria cycle and cancer terms in questions on common diseases in humans.

1. Biology in Human Welfare

1.1 Biology and human welfare

• Biology is the youngest of the formalised disciplines of natural science.
• Physics and chemistry progressed much faster, and their uses in daily life are more visible than those of biology.
• The twentieth and twenty-first centuries showed the value of biological knowledge for human welfare, in health and in agriculture.
• ★ Exam imp The discovery of antibiotics, synthetic plant-derived drugs and anaesthetics changed medical practice and human health.
• Life expectancy of human beings has changed dramatically over the years.
• Agricultural practices, food processing and diagnostics have brought socio-cultural changes in human communities.

1.2 M. S. Swaminathan (1925-2023)

• Monkambu Sambasivan Swaminathan was born in August 1925 in Kumbakonam, Tamil Nadu.
• He did his graduation and post-graduation in Botany at Madras University, and built expertise in genetics and plant breeding.
• He worked in many institutions in India and abroad, in different capacities.
• At the School of Cytogenetics and Radiation Research, Indian Agricultural Research Institute (IARI), his team developed short-duration, high-yielding rice varieties, including scented Basmati.
• He developed the concepts of crop cafeteria and crop scheduling, and genetically improved crop yield and quality.
• ★ Exam imp His collaboration with Norman Borlaug led to the Green Revolution in India, through Mexican varieties of wheat; this work was highly recognised and appreciated.
• He initiated 'Lab-to-Land', food security and several environmental programmes; his honours include the Padma Bhushan and several other prestigious awards, medals and fellowships.

2. Health

2.1 Old ideas and new evidence

• For a long time, health was thought to be a state of body and mind with a balance of certain 'humors'.
• Early Greeks such as Hippocrates, and the Indian Ayurveda system of medicine, held this view.
• Persons with 'blackbile' were believed to have a hot personality and to suffer fevers; this idea came from pure reflective thought.
• ★ Exam imp William Harvey discovered blood circulation by the experimental method. The thermometer showed normal body temperature in persons with blackbile. Together, these disproved the 'good humor' hypothesis.
• Later, biology showed that the mind influences the immune system through the neural and endocrine systems.
• The immune system maintains our health, so the mind and mental state can affect health.

2.2 Factors that affect health

• Genetic disorders: deficiencies a child is born with, and defects inherited from the parents from birth.
• Infections.
• Life style: the food and water we take, the rest and exercise we give our bodies, and the habits we have or lack.

2.3 What health means

• Healthy people are more efficient at work; this increases productivity and brings economic prosperity.
• Health also increases longevity and reduces infant and maternal mortality.

2.4 Maintaining good health

• A balanced diet, personal hygiene and regular exercise are very important.
• Yoga has been practised since time immemorial to achieve physical and mental health.
• Also necessary: awareness about diseases and their effects on body functions, vaccination (immunisation) against infectious diseases, proper disposal of wastes, control of vectors, and hygiene in food and water resources.

3. Disease and Pathogens

3.1 Disease

• Disease: the functioning of one or more organs or systems of the body is adversely affected, with various signs and symptoms.
• Diseases are broadly grouped into infectious and non-infectious.
• Infectious diseases: diseases easily transmitted from one person to another. They are very common, and every one of us suffers from them at some time or other; some, like AIDS, are fatal.
• ★ Exam imp Among non-infectious diseases, cancer is the major cause of death.
• Drug and alcohol abuse also affect our health adversely.

3.2 Pathogens

• Pathogens: disease-causing organisms. They belong to bacteria, viruses, fungi, protozoans, helminths, etc.
• Most parasites are pathogens, because they harm the host by living in or on it.
• Pathogens enter the body, multiply and interfere with normal vital activities, causing morphological and functional damage.
• Pathogens must adapt to life inside the host. For example, gut pathogens must survive the low pH of the stomach and resist digestive enzymes.

4. Bacterial Diseases: Typhoid and Pneumonia

4.1 Typhoid

• ★ Exam imp Salmonella typhi: a pathogenic bacterium that causes typhoid fever in human beings.
• It enters the small intestine through contaminated food and water, then migrates to other organs through the blood.
• Symptoms: sustained high fever ($39$ to $40{}^{\circ }\mathrm{C}$), weakness, stomach pain, constipation, headache and loss of appetite.
• In severe cases, intestinal perforation and death may occur.
• ★ Exam imp Typhoid fever is confirmed by the Widal test.
• Mary Mallon, nicknamed Typhoid Mary, was a cook and a typhoid carrier. She spread typhoid for several years through the food she prepared.

4.2 Pneumonia

• Caused by the bacteria Streptococcus pneumoniae and Haemophilus influenzae.
• ★ Exam imp They infect the alveoli (air-filled sacs) of the lungs. The alveoli fill with fluid, causing severe problems in respiration.
• Symptoms: fever, chills, cough and headache. In severe cases, the lips and finger nails turn grey to bluish.
• Spread: a healthy person inhales droplets or aerosols released by an infected person, or shares glasses and utensils with one.

4.3 Other bacterial diseases

• Dysentery, plague and diphtheria are some other bacterial diseases in man.

5. Viral Disease: Common Cold

• Many viruses cause diseases in human beings.
• Rhino viruses cause the common cold, one of the most infectious human ailments.
• ★ Exam imp They infect the nose and respiratory passage, but not the lungs.
• Symptoms: nasal congestion and discharge, sore throat, hoarseness, cough, headache and tiredness; they usually last 3-7 days.
• Spread: droplets from the cough or sneeze of an infected person are inhaled directly, or picked up from contaminated objects such as pens, books, cups, doorknobs, computer keyboard or mouse.

6. Protozoan Diseases: Malaria and Amoebiasis

6.1 Malaria

• Malaria is a disease man has been fighting for many years. It is caused by Plasmodium, a tiny protozoan.
• ★ Exam imp Different species of Plasmodium cause different types of malaria: P. vivax, P. malariae and P. falciparum.
• ★ Exam imp Malignant malaria, caused by Plasmodium falciparum, is the most serious one and can even be fatal.

6.2 Life cycle of Plasmodium

1. Plasmodium enters the human body as sporozoites (the infectious form) through the bite of an infected female Anopheles mosquito.
2. The sporozoites reach the liver through the blood. The parasites first multiply asexually within liver cells, which burst and release them into the blood.
3. The parasites then attack red blood cells (RBCs), multiply asexually inside them and rupture them. The released parasites infect new RBCs.
4. Rupture of RBCs releases a toxic substance, haemozoin, which causes the chill and high fever recurring every three to four days.
5. Sexual stages, the male and female gametocytes, develop in red blood cells.
6. A female Anopheles mosquito bites an infected person and takes up the gametocytes with its blood meal.
7. Fertilisation and development take place in the mosquito's gut.
8. The parasites multiply to form sporozoites, which escape from the gut and are stored in the mosquito's salivary glands.
9. When this mosquito bites a human, the sporozoites are introduced into the body, and the events begin again.

6.3 Amoebiasis

• Entamoeba histolytica: a protozoan parasite in the large intestine of humans; it causes amoebiasis (amoebic dysentery).
• Symptoms: constipation, abdominal pain and cramps, and stools with excess mucus and blood clots.
• ★ Exam imp Houseflies act as mechanical carriers. They carry the parasite from the faeces of an infected person to food and food products, contaminating them.
• Drinking water and food contaminated by faecal matter are the main source of infection.

7. Helminth Diseases: Ascariasis and Filariasis

• Ascaris (the common round worm) and Wuchereria (the filarial worm) are helminths known to be pathogenic to man.

7.1 Ascariasis

• Ascaris is an intestinal parasite that causes ascariasis.
• Symptoms: internal bleeding, muscular pain, fever, anemia and blockage of the intestinal passage.
• The eggs of the parasite pass out with the faeces of infected persons and contaminate soil, water, plants, etc.
• ★ Exam imp A healthy person gets the infection through contaminated water, vegetables and fruits.

7.2 Filariasis (elephantiasis)

• ★ Exam imp Caused by Wuchereria (W. bancrofti and W. malayi), the filarial worms.
• They cause a slowly developing, chronic inflammation of the organs in which they live for many years, usually the lymphatic vessels of the lower limbs.
• The disease is called elephantiasis or filariasis.
• The genital organs are also often affected, resulting in gross deformities.
• ★ Exam imp The pathogens are transmitted to a healthy person through the bite of female mosquito vectors.

8. Fungal Disease: Ringworm

• ★ Exam imp Many fungi of the genera Microsporum, Trichophyton and Epidermophyton cause ringworms.
• Ringworm is one of the most common infectious diseases in man.
• ★ Exam imp Main symptoms: dry, scaly lesions on parts of the body such as the skin, nails and scalp, with intense itching.
• Heat and moisture help these fungi grow, so they thrive in skin folds, such as those in the groin or between the toes.
• Ringworms are generally acquired from soil, or by using the towels, clothes or even the comb of infected individuals.

9. Prevention and Control of Infectious Diseases

9.1 Hygiene

• Personal and public hygiene are very important for preventing and controlling many infectious diseases.
• Personal hygiene: keeping the body clean; consuming clean drinking water, food, vegetables, fruits, etc.
• Public hygiene: proper disposal of waste and excreta; periodic cleaning and disinfection of water reservoirs, pools, cesspools and tanks; standard practices of hygiene in public catering.
• ★ Exam imp These measures are especially essential for diseases spread through food and water: typhoid, amoebiasis and ascariasis.
• Air-borne diseases such as pneumonia and the common cold: in addition, avoid close contact with infected persons or their belongings.

9.2 Control of vectors

• For diseases spread by insect vectors, such as malaria and filariasis, the most important measure is to control or eliminate the vectors and their breeding places.
• Avoid stagnation of water in and around residential areas.
• Clean household coolers regularly, and use mosquito nets.
• ★ Exam imp Introduce fishes like Gambusia in ponds; they feed on mosquito larvae.
• Spray insecticides in ditches, drainage areas and swamps.
• Provide doors and windows with wire mesh to prevent the entry of mosquitoes.
• These precautions matter more because of recent widespread vector-borne diseases spread by Aedes mosquitoes: dengue and chikungunya, in many parts of India.

9.3 Vaccines and drugs

• ★ Exam imp Vaccines and immunisation programmes have completely eradicated smallpox.
• Polio, diphtheria, pneumonia and tetanus have been controlled to a large extent by vaccines.
• Biotechnology is on the verge of making newer and safer vaccines available.
• The discovery of antibiotics and other drugs has also made it possible to treat infectious diseases effectively.

10. Cancer

10.1 Cancer: an overview

• Cancer is one of the most dreaded diseases of human beings and a major cause of death all over the globe.
• More than a million Indians suffer from cancer, and a large number of them die from it every year.
• The mechanisms behind oncogenic transformation of cells, and the treatment and control of cancer, are among the most intense areas of research in biology and medicine.

10.2 Normal cells and cancer cells

• In the body, cell growth and differentiation are highly controlled and regulated.
• In cancer cells, these regulatory mechanisms break down.
• Cancer cells keep dividing and give rise to masses of cells called tumors.

10.3 Benign and malignant tumors

10.4 Causes of cancer

• Carcinogens: physical, chemical or biological agents that transform normal cells into cancerous neoplastic cells.
• ★ Exam imp Physical: ionising radiations such as X-rays and gamma rays, and non-ionising radiations such as UV, damage DNA and lead to neoplastic transformation.
• ★ Exam imp Chemical: the chemical carcinogens in tobacco smoke are a major cause of lung cancer.
• Biological: cancer-causing viruses, called oncogenic viruses, have genes called viral oncogenes.
• ★ Exam imp Cellular oncogenes (c-onc) or proto oncogenes: genes found in normal cells which, when activated under certain conditions, can lead to oncogenic transformation.

10.5 Detection and diagnosis

• Early detection is essential, because it allows the disease to be treated successfully in many cases.
• ★ Exam imp Biopsy: a piece of the suspected tissue is cut into thin sections, stained and examined under a microscope by a pathologist (histopathological studies).
• Leukemias: detected by blood and bone marrow tests for increased cell counts.
• Cancers of internal organs: radiography (use of X-rays), CT (computed tomography) and MRI (magnetic resonance imaging) are very useful.

• Antibodies against cancer-specific antigens are also used to detect certain cancers.
• Molecular biology techniques can detect genes that give an inherited susceptibility to certain cancers.
• Identifying such genes, which predispose a person to certain cancers, can be very helpful in preventing cancer.
• Such individuals may be advised to avoid the particular carcinogens they are susceptible to, for example tobacco smoke in the case of lung cancer.

10.6 Treatment of cancer

• The common approaches are surgery, radiation therapy, chemotherapy and immunotherapy.
• Radiotherapy: tumor cells are irradiated lethally, taking proper care of the normal tissues surrounding the tumor mass.
• Chemotherapy: several chemotherapeutic drugs are used to kill cancerous cells; some are specific for particular tumors.
• The majority of these drugs have side effects such as hair loss and anemia.
• Most cancers are treated by a combination of surgery, radiotherapy and chemotherapy.
• ★ Exam imp Immunotherapy: tumor cells avoid detection and destruction by the immune system. So patients are given biological response modifiers such as $\alpha$-interferon, which activates their immune system and helps destroy the tumor.

11. Exam Essentials

Diseases at a Glance

Pairs to Match

Numbers to Remember

• Typhoid fever: sustained $39$ to $40{}^{\circ }\mathrm{C}$.
• Common cold symptoms last 3-7 days.
• Malarial chill and fever recur every 3 to 4 days.
• Plasmodium: 3 species named; 2 hosts.
• Wuchereria: 2 species; ringworm fungi: 3 genera.
• Factors affecting health: 3 (genetic disorders, infections, life style).
• Cancer: more than a million Indians suffer from it.
• M. S. Swaminathan: born August 1925 in Kumbakonam.

Examples to Remember

12. Quick Revision

• Antibiotics, plant-derived drugs and anaesthetics changed medicine; Swaminathan and Borlaug brought the Green Revolution.
• Harvey's experiments and the thermometer disproved the 'good humor' hypothesis.
• Health: complete physical, mental and social well-being; affected by genetic disorders, infections and life style.
• Pathogens: bacteria, viruses, fungi, protozoans and helminths; typhoid, cholera, pneumonia, fungal skin infections and malaria cause much distress.
• Typhoid: Salmonella typhi, small intestine, $39$ to $40{}^{\circ }\mathrm{C}$ fever, Widal test, Typhoid Mary.
• Pneumonia: S. pneumoniae and H. influenzae; alveoli fill with fluid; grey to bluish lips and nails.
• Common cold: rhino viruses; nose and respiratory passage only; 3-7 days.
• Malaria: sporozoites, liver, RBCs, haemozoin, gametocytes; two hosts; female Anopheles is the vector.
• Amoebiasis: Entamoeba histolytica in the large intestine; houseflies are mechanical carriers.
• Ascariasis: Ascaris; filariasis: Wuchereria, lymphatic vessels of the lower limbs.
• Ringworm: Microsporum, Trichophyton, Epidermophyton; dry scaly itchy lesions.
• Prevention: personal cleanliness, waste disposal, decontamination of drinking water, vector control (Gambusia, nets, wire mesh), vaccines and antibiotics; smallpox eradicated.
• Cancer: loss of contact inhibition; benign vs malignant tumors; metastasis.
• Carcinogens: X-rays, gamma rays, UV, tobacco smoke, oncogenic viruses; proto oncogenes (c-onc).
• Detection: biopsy, blood and bone marrow tests, radiography, CT, MRI, antibodies; treatment: surgery, radiotherapy, chemotherapy, immunotherapy.

13. Solved Examples

14. Practice Questions

Common Mistakes to Avoid