Viruses are different from other microbes. They are very small. They have no cellular structure and are obligate intracellular parasites. They can only multiply inside the living cells of animals or plants or other microbes. This process harms the host, resulting in a disease. Outside of the host they are inert particles called virions.

A basic viral particle consists of a core of nucleic acid, which can either be single or double stranded DNA or RNA but not both. A protein coat called a capsid surrounds the nucleic acid and is made of building blocks called capsomers. Some viruses have an envelope around the capsid. There are three virus shapes: polyhedral, rod and complex.

Bacteriophages are viruses that infect bacteria. They are used in gene technology to transfer foreign DNA that has been spliced into their nucleic acid into bacterial cells. The bacteria then acquire the ability to carry out the function of that particular gene and make specific proteins.