Hidden inside all of us are likely thousands of viruses — maybe more. They just hang out, harmlessly. We don't even know they're there.
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.
The findings, published in the journal Science Translational Medicine, add to growing evidence that some viruses may help calibrate the immune system. They tell immune cells which pathogens to assault and which ones to leave alone.
Now, we're not talking about a rare virus that only a few people harbor. 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.
CMV likes to hang out in salivary glands. So it's passed from person to person through saliva (and other bodily fluids). Most of us pick up CMV when we're toddlers and like to put toys, keys — well, just about everything — in our mouths.
"Billions of people are infected with CMV throughout their lifetime and apparently suffer nothing," says the study's senior author, Mark Davis. "We wanted to know why it's so widespread. Why doesn't the body do more to get rid of it? Our result surprised us."
Previous studies have hinted that CMV damages the immune system as we age. So Davis and his team looked to see if elderly people with CMV had a weaker immune reaction to the flu vaccine than those not infected.
"We didn't see anything going on with people over age 60," Davis says. "But when we looked at people age 20 to 30, we were like, 'Oh! There's something there.' "
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," 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.
Davis says the goal is to find out how CMV protects against the flu and use its tricks to build a better vaccine. "Then we could boost people's immune systems without having the problems that we get by putting a virus in them," he says.
Immunologist Erik Barton of Wake Forest School of Medicine also thinks the findings may help scientists develop better vaccines.
But Barton, who wasn't involved with study, agrees we need to be careful with CMV. He thinks that, like many bacteria in our gut, CMV is a Janus-faced inhabitant: Sometimes its effects are positive. Sometimes they might be negative, even when we're in good health.
"CMV is most likely to help out the immune system when we're young kids, and we get the most infections," says Barton. "Then CMV may protect us from the next flu pandemic."
As we age, though, CMV may turn harmful. Studies have correlated CMV with a host of problems later in life, such as autoimmune diseases and heart disease.
"But it's all been guilt by association," Barton says. "CMV and related viruses boost parts of the immune system known to be involved in these diseases. But we still don't know if the viruses are the cause."
One idea is clear though, he says: Viruses are no longer the clear enemy. Sometimes they're even our friends.
"We've viewed viruses for so long as pathogens and as bad," Barton says."But we've never actually done studies to see if they're doing something beneficial. This study is just the tip of the iceberg of what's possible."