This is a freaky one. You sometimes hear that a virus will “die” without a host, or after a certain amount of time outside the body. But there’s no real consensus that viruses are alive in the first place.
Let’s do a little comparison of two well-known disease-causing agents to illuminate the issue. Bacteria are one-celled organisms that grow, replicate themselves, respond to stimuli, maintain internal homeostasis and metabolize “food” for energy. Viruses, on the other hand, do none of those things. They’re basically just genetic material wrapped in a protein coat. They’re very, very good at infecting cells, and they do this because they need the cells to replicate more viruses. This inability to reproduce themselves is one reason for the argument that viruses are not alive. At the same time, that they reproduce at all is an argument that they are, in fact, living.
Many scientists believe that viruses exist at the very edge of our concept of “alive.” I was talking to Peter Kasson, PhD, of our Department of Molecular Physiology and Biological Physics, the other day, and this was the answer he gave me when I asked him about it: Viruses straddle our notion of “living” and “nonliving.”
A couple of years ago, a team led by scientists at the University of Illinois at Urbana-Champaign published an analysis arguing, in essence, that the case for “living” is bolstered by the evolution of viruses from the days before either cells or viruses as we know them existed. The researchers identified 442 protein folds shared between cells and viruses, while 66 folds were unique to viruses. That last part is key, in that it suggests viruses aren’t just nonliving debris from living cells. Instead, the findings suggest, “viruses originated from multiple ancient cells … and co-existed with the ancestors of modern cells,” the researchers wrote.
You can read more about it here, but, in the end, you’ll have to make up your own mind.