A virus is a tiny infectious agent that can only replicate inside the living cells of an organism. It consists of genetic material — either DNA or RNA — surrounded by a protective protein coat called a capsid. Some viruses also have an outer lipid envelope. Viruses cannot reproduce or carry out metabolic processes on their own and depend entirely on a host cell's machinery to make copies of themselves. They infect all forms of life, including animals, plants, fungi, bacteria, and archaea. When a virus infects a host cell, it often redirects the cell's processes to produce many copies of the virus, which can then go on to infect other cells. Viruses are responsible for a wide range of diseases, such as the flu, COVID-19, and many others.