Any of a variety of micro-organisms that can infect cells and cause disease. Viruses use the chemicals inside cells they infect to keep themselves alive and multiply (replicate). They may contain DNA or RNA as their genetic material.

Viruses cause a number of infectious diseases and may also cause some types of cancer. For example, infection with the human papillomavirus (HPV) is associated with an increased risk of cervical cancer.

Viral means referring to or having to do with a virus or its effects, as in viral hepatitis.