The herpes family of viruses can have a surprising upside--it can protect against the bubonic plague and other bacterial contagions, at least in mice.

...

"There may be symbiotic advantages to chronic infections with these viruses."

...

These new results do not mean people should go out and get infected with herpes, Virgin stressed. They probably already are. Nearly all humans become infected with multiple herpes virus family members during childhood. These germs not only include the herpes simplex viruses, which lead to cold sores and possibly genital herpes, but also the diseases responsible for chickenpox and "mono," as well as several less well-known ailments. Herpes infections have bedeviled animals for more than 100 million years.