Flexi Says: A virus is essentially DNA or RNA surrounded by a coat of protein (Figure below). It is not made of a cell, and cannot maintain a stable internal environment (homeostasis). Recall that a cell is the basic unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce on their own—they need to infect a host cell to reproduce. So a virus is very different from any of the organisms that fall into the three domains of life. Though viruses are not considered living, they share two important traits with living organisms. They have genetic material like all cells do (though they are not made of cells), and they can evolve. The genetic material of a virus can change (mutate), altering the traits of the virus. As the process of evolution has resulted in all life on the planet today, the classification of viruses has been controversial. It calls into question the very definition of life.