Microorganisms

Microorganisms - Microscopic are living organisms (too small to be seen with the naked eye). Many can grow in or on a host organism and cause disease.

Types: bacteria, viruses, fungi and protozoa

Bacteria - A very small single cell organism. It has a cell wall and an atypical nucleus that lacks a membrane. It can replicate without a host cell.

Classified according to shape: spherical (Cocci), rod-shaped (Bacilli) and spiral (spirilla or spirochetes).

Virus - A small infectious agent that can replicate only inside the living cells of an organism (a virus cannot live outside a living cell).

Fungi (fungus is singular) - Can be single-celled, multinucleated or multicellular organisms. They include yeasts, molds and mushrooms.

Protozoa (protozoon is singular) - A subkingdom comprising the simplest organisms of the animal kingdom, consisting of unicellular organisms ranging in size from submicroscopic to macroscopic.