Biological Agents - Viruses
Viruses are the simplest microorganisms and are much smaller than bacteria. Viruses are intracellular parasites and lack a system for their own metabolism; therefore, they invade host cells to live. Viruses, unlike bacteria, require living host cells in order to reproduce. The host cells can be human, animal, plant or bacteria. A virus typically brings about changes in the host cell that eventually lead to cell death.