There's nothing unusual about a particular species harboring a particular virus. Mice, ticks, mosquitoes, plants and people all play host to certain viruses that constantly evolve in their own struggle for survival. This is nature at work. On its own, a virus isn't even strictly alive, since it doesn't have the means to reproduce or to propel itself. Yet viruses are marvels of evolutionary efficiency.
Viruses inject their genetic instructions into infected cells, commandeering their machinery.
Some of the worst human infectious diseases are caused by viruses, including AIDS, hepatitis, polio and influenza. But viruses also infect plants, insects and even bacteria. How dangerous a virus is depends on many factors, including how easily it spreads, what kind of cells it targets and how hearty it is. No sooner do we become immune to one strain than a new one appears that our immune systems can't recognize. That's why health experts must create a new influenza vaccine each year.