Diseases, where microbes are the immediate causes, are called infectious disease. The agents like microbes can spread in the community and the disease they cause will spread with them. Such disease-causing microbes are called’ infectious agents.
The following infections agents are:
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.
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 a common cold, influenza, dengue fever, SARS, AIDS, measles, mumps, polio, smallpox, chicken pox etc.
Protozoans: They are microscopic unicellular, eukaryotic organisms. They can reproduce of their own. The various diseases caused by protozoa are malaria by Plasmodium, Kala-azar etc.
Helminths: Helminthes are multicellular worms which are mostly present in the intestine. They cause taeniasis, ascariasis, elephantiasis worm, hence also known as filariasis etc.
Fungi: They are also multicellular eukaryotic, heterotrophic organisms. They cause ringworm, athlete’s foot and other skin infections.
