There are five major groups of micro-organisms. These are: Bacteria, viruses, Protozoa, some fungi and Algae.

(a) Bacteria − They are single celled disease-causing micro-organisms. They can be spiral or rod- shaped. Examples- Lactobacillus bacteria.

(b) Virus −These are disease-causing microbes that reproduce only inside the host organism. Example- HIV.

(c) Protozoa − They can be unicellular or multicellular. Example- Plasmodium.

(d) Fungi − They are mostly multicellular. Bread moulds are common examples of fungi.

(e) Algae − They include multicellular, photosynthetic organisms such as Spirogyra, Chlamydomonas, etc.