Class 12 Biology

Human Health and Disease

Health: A state of complete physical, mental and social well being is called health.

Disease: It is a condition in which one or more organs or systems of the body are adversely affected. Disease is characterized by various signs and symptoms.

Infectious Disease: A disease which can be easily transmitted from one person to another is called infectious disease.

Non-infectious Disease: A disease which cannot be transmitted from one person to another is called non-infectious disease.

Pathogens: Organisms which cause diseases are called pathogens. Examples: bacteria, viruses, fungi, protozoans, helminths, etc.

Common Diseases in Humans


This disease is caused by a bacterium called Salmonella typhi. This disease spreads through contaminated water. Common symptoms of this disease are; sustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache and loss of appetite. Typhoid fever can be confirmed by Widal test.


This disease is caused by bacteria; like Streptococcus pneumoniae and Haemophilus influenza. These bacteria infect the alveoli in lungs. Due to infection, the alveoli get filled with fluid; leading to severe problems in respiration. Some of the symptoms of pneumonia are; fever, chills, cough and headache, lips and finger nails may become bluish in severe cases. This disease is spread through droplets in air. When an infected person sneezes or coughs, he releases these droplets in air. It can also spread by sharing glasses and utensils with the infected person.

Common cold

This disease is caused by Rhino viruses, and spreads through droplets in air. Nose and respiratory passage are infected in common cold. Some of the symptoms of common cold are; nasal congestion, discharge, sore throat, hoarseness, cough, headache, tiredness, etc. A person can also catch common cold through contaminated objects; like pen, book, doorknob, keyboard, mouse, etc.


This disease is caused by protzoans. Different species of Plasmodium (P. vivax, P. malaria and P. falciparium) cause different types of malaria. Malignant malaria is caused by Plasmodium falciparum. This is the most serious type of malaria and can even prove fatal.

Life Cycle of Plasmodium


This disease is caused by a protozoan; called Entamoeba histolytica. This parasite lives in the large intestine of humans. Symptoms of amoebiasis are; constipation, abdominal pain, cramps, stool with excess mucus and blood clots. Houseflies are the mechanical carriers of this parasite and transmit the parasite from faeces of infected person to food and water. Thus, this disease spreads through contaminated food and water.


This disease is caused by the common roundworm Ascaris. This is an intestinal parasite. Symptoms of this disease are; internal bleeding, muscular pain, fever, anemia and blockage of intestinal passage. The eggs of the parasite are released along with the faeces of infected person. The eggs then contaminate soil, water, plants, etc. A healthy person may get infected through contaminated water, vegetables, fruits, etc.


This disease is caused by the filarial worm Wuchereria (W.bancrofti and W. malayi). The infected person slowly develops chronic inflammation of the affected organs. The usual sites are lymphatic vessels of the lower limbs. This disease is also known as elephantiasis. Genital organs also get infected; resulting in gross deformity. This disease spreads through the bite of female mosquito vectors.


This disease is caused by many fungi of the genera Microsporum, Trichophyton and Epidermophyton. The symptoms of this disease are dry, scaly lesions on various parts of the body such as skin, nails and scalp. The lesions are accompanied by intense itching. Heat and moisture help these fungi to grow. Thus, the fungi thrive in skin folds such as those in the groin or between the toes. This disease spreads through soil or through contaminated clothes or even comb.

Prevention of Infectious Diseases

Personal and public hygiene are important to prevent and control many infectious diseases. A person should maintain cleanliness of the body. Water, food, vegetables, fruits, etc. should be kept contamination free. Proper disposal of waste and excreta is important for maintaining public hygiene. Water reservoirs, pools, cesspools and tanks should be cleaned and disinfected at regular intervals. Vector borne disease can be prevented by checking the breeding of vectors; such as mosquito. Using mosquito repellants and mosquito net also helps.