We often assume that viral infections are caused by individual virus types. But in reality, we’re exposed to many viruses on a day-to-day basis, and co-infection – where someone is simultaneously infected by two or more virus types – is quite common. The cells lining our throat and lower airways are exposed to the environment around us, making them a prime target for co-infection by respiratory viruses. These range from common-cold-causing rhinoviruses to the more serious influenza viruses, which are often the cause of global pandemics. Viral interference One of the most frequent outcomes of co-infection is viral interference, a phenomenon where one virus out-competes and suppresses the replication of the other co-infecting viruses.