The smallest of all life forms, viruses are microscopic infectious agents that infect many types of living cells. They are found everywhere, in every ecosystem on Earth. Viruses are distinct from bacteria and other single-celled microorganisms (such as fungus and protozoa) in that they cannot reproduce on their own. They occupy a gray area on the “tree of life” that blurs the distinction between living and nonliving, as they can only replicate by harnessing a host cell’s replication mechanisms.

A virus consists of a core of genetic material (either DNA or RNA) surrounded by a protein shell known as a capsid. The proteins that comprise the capsid can vary in shape, size and complexity. This is how viruses can take on a range of shapes, from short and highly rigid rods to long, flexible filaments. The size of a virus is determined by the length of its core of DNA or RNA. Viruses are also unique in that they are encased in a lipid membrane, allowing them to cross the plasma membrane and enter living cells.

During the lytic cycle, a virus splinters into pieces of its own genetic material and then uses the host cell’s machinery to make more copies of itself. When enough viruses have accumulated, the host cell’s metabolism and protein-synthesis processes are overwhelmed and it bursts open, spreading the viral particles into the body to infect more cells.

Viruses infect many different types of living cells, and viruses are very important to the study of molecular and cell biology. They provide simple systems for studying aspects of cell replication, transcription, translation, and protein transport. They have even been used to introduce new genes into cells, which has led to important advances in molecular genetics and biotechnology.