We go back a long way with herpesviruses. Our evolutionary line has been living with these genomic parasites for more than 100 million years, and today herpesviruses infect nearly all humans, as well as all other mammals, birds and reptiles that scientists have checked. We aren't born with these viruses, but most of us acquire multiple infections of various types of herpesviruses during childhood.

Unlike our relationship with many other, more notorious viruses, we've learned to peacefully coexist with herpesviruses for the most part. They set up shop in our cells, they use our molecular machinery to replicate themselves, and they take advantage of the influx of energy we provide from our diet. Sometimes the relationship goes sour, with the unfortunate results ranging from chickenpox to mononucleosis, genital herpes, and Burkitt's lymphoma, but by and large, most humans, and mammals in general, are never seriously harmed by these house guests. But do we get anything out of this relationship? Remarkably, recent research suggests that this 100 million-year coexistence may have been good for us too, helping our immune system to ward off even more serious pathogens.