• are very small and can only be seen under an electron microscope
• have no cellular structure
• are obligate intracellular parasites, which means they can only multiply inside the living cells of animals, plants or other microbes. This process harms the host, resulting in a disease. Outside of the host they are inert particles called virions.

Examples of virus diseases in humans are measles, chicken pox, 'flu and AIDS.

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.

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 up of building blocks called capsomers. Some viruses have an envelope around the capsid. There are three virus shapes: polyhedral, rod and complex.