Bacteria are unicellular, prokaryotic micro-organism found almost in all kinds of habitats. It exists in a various sizes and shapes. Some bacteria are beneficial like those involved in Nitrogen Fixation and some are pathogenic cause diseases like cholera, tuberculosis and anthrax etc. Cocci, Spirilla and Bacilli are the names given according to its various shapes.
Virus is a unicellular tiny organism which is mostly composed of DNA and protein. Its body comprises head and tail sections. Viruses get attached themselves to other organism and causes severe infectious diseases. Commonly known viral diseases include flu, AIDS and Hepatitis etc.
Fungi are a group of simple plants whose cells are devoid of chlorophyll. Fungus has chitin in its cell wall instead of cellulose. Fungi are popular for beneficial effects including food production, penicillin production and decomposition etc. Commonly known fungi include yeast, mold and mushrooms etc.
Parasites are those micro-organisms which depend on other organisms (host) for their survival. Both virus and bacteria could be parasites. Commonly known parasite is hookworm, which attaches with intestinal lining and causes digestive diseases including malnutrition etc.