Infectious agents: Disease where microbes are the immediate causes are called infectious diseases.

The infectious agents, like microbes, can spread in the community and the disease they cause, will spread; them.

The following infections agents are:

1. Bacteria: They are unicellular, prokaryotic, microscopic organisms. They reproduce very quickly. Some common diseases caused by bacteria are typhoid, cholera, tuberculosis, anthrax, diphtheria, tetanus, etc.

2. Viruses: They are submicroscopic organisms. They cannot reproduce by themselves, because they do not have their own metabolic machinery. They utilise the metabolic machinery of the host cell and multiply. The various diseases caused by viruses are common cold, influenza, dengue fever, SARS, AIDS, measles, mumps, polio, small pox, chicken pox, etc.

3. Protozoans: They are microscopic, unicellular, eukaryotic organisms. They can reproduce on their own. The various diseases caused by protozoa are malaria, by Plasmodium, Kala-azar, etc.

4. Helminths: Helminthes are multicellular worms, which are mostly present in the intestine. They cause taeniasis, ascariasis, elephantiasis worm, hence also known as filariasis, etc.

5. Fungi: They are also multicellular, eukaryotic, heterotrophic organisms. They cause ringworm, athlete’s foot id other skin infections.