New research done at two English universities found that viruses can easily and transiently develop the ability to adjust their behavior based on how many other viruses there currently are in their host. This means that viruses don't infect a host on its own; it can work in conjunction with other viruses to maximize its spread (and its damage on the host population).
Furthermore, researchers found that the virus would only cooperate with similar viruses, i.e. viruses it was related to. When it infected alone, it would clone itself and then work with the newly cloned viruses to. When this was the case, all clones worked together to kill the host as slowly as possible to enable maximum replication of the virus. But when other viruses, to which it was not related, were present in the cell, it deliberately worked against them to kill the host cell faster so it could dominate the other viruses.
The study looked at a relatively simple virus, and thus the astonishing speed at which it can adapt its behavior is especially discouraging. Humans have been on a never-ending quest to evade/better the viruses like influenza that infect us, but this news tells us that we have a long way to go. The evolutionary arms race rages on.