But every once in a while, one of these viral inhabitants might help us out.

Young people infected with a type of herpes virus have a better immune response to the flu vaccine than those not infected, scientists at Stanford University report Wednesday. In mice, the virus directly stops influenza itself.
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We’re talking about a ubiquitous critter, called cytomegalovirus. About half of all Americans carry it. And so do nearly 100 percent of people in developing countries.
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In younger people, CMV had the opposite effect that Davis had predicted: “The virus ramped up the immune system to give better protection from pathogens,” Mark Davis says. “We tested only for the flu, but I speculate it protects against everything.”

So should we all go out and get infected with CMV? No way! Davis exclaims.

You see, CMV has a dark side. It can become dangerous if the immune system is suppressed, which happens after an organ transplant or during treatments for autoimmune disorders. CMV is also a concern for pregnant woman. It’s the top viral cause of birth defects worldwide.