There's about a 50 percent chance you already have cytomegalovirus (CMV), but don't worry, it's pretty harmless in healthy adults. It's long been believed that the virus probably weakens the immune system in older people, but now mice tests have shown that the opposite is true – somehow, the virus gives the immune system a bit of a late-stage boost.

Those infected with CMV might feel like the bug has passed after the initial minor flu-like symptoms fade away, but as a member of the herpes family, it'll stick with you for life. Given the prevalence of the virus, researchers set out to investigate its effects on the immune system of elderly people.

"CMV doesn't usually cause outward symptoms, but we still have to live with it every day since there's no cure," says Megan Smithey, an author of the study. "Our immune system always will be busy in the background dealing with this virus."

To study the bug, the team infected older mice with CMV, then introduced listeria to see how they would cope. Interestingly, the CMV-affected mice actually managed to fight off the new virus better than a control group without CMV.

"We assumed it would make mice more vulnerable to other infections because it was using up resources and keeping the immune system busy," says Smithey. "We were completely surprised; we expected these mice to be worse off. But they had a more robust, effective response to the infection."

While the researchers aren't sure exactly how CMV helps, they did spot clues in the diversity of their T-cells, the foot soldiers of the immune system. The more types of T-cells running around in the body, the better prepared the immune system is to fight off a range of different infections, and this diversity has long been thought to naturally drop off as we age.

But the researchers found that both groups of mice had fairly diverse T-cells. Instead, the perceived dropoff is because not all of them are charging into battle against infection. CMV seems to help with that, somehow.

"It's as if CMV is issuing a signal that gets the best defenses out onto the field," says Janko Nikolich-Žugich, an author of the study.

In future work, the team plans to investigate how CMV is strengthening the immune system, and check whether the phenomenon also applies to human studies. If so, the long-term goal could be a kind of "booster shot" for the immune system.

The research was published in the Proceedings of the National Academy of Sciences.

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