Micro-organisms like bacteria, fungi, protozoa and viruses cause many diseases in human beings. They enter our body through contaminated water, food or milk, through contaminated air and insect bites. Dirty open drains, stagnant pools of water and garbage dumps are ideal places for the micro-organisms and insects like mosquitoes and flies to grow and breed. The following table gives the diseases caused by different micro-organisms in human beings:
Malaria is caused by a protozoan plasmodium. It is spread by female anopheles mosquito. When the mosquito bites a person to suck blood, it injects plasmodium into his/her blood along with its saliva. Plasmodium multiplies inside the human body and causes fever. Malaria is characterized by high fever and shivering. Female anopheles mosquito is thus said to be the carrier of the disease.
Dengue is another dreaded disease spread by Aedes mosquito. It is caused by a virus and spread by the bite of Aedes mosquito.
Plague, a dreaded epidemic disease, is caused by a bacterium found in the body of rat fleas. Rat fleas live on the body of rats and mice. When a flea bites human beings or they accidentally come into contact with infected rats, the bacterium enters human body and causes plague.
Insects like cockroaches and houseflies are carriers of many diseases. They visit garbage dumps, human/animal wastes and drains and the disease- causing micro-organisms cling to their claws and legs. When these insects come in contact with our food, they drop numerous bacteria on it. Flies have sticky hairs on their legs which carry bacteria and other germs from place to place. Flies are the carriers of several diseases like cholera, typhoid, tuberculosis and diarrhoea.