ST. LOUIS – A case of mononucleosis now might save you from food poisoning later, according to new research from Washington University.

Scientists say that if the results from their research with mice hold true for humans, it could redefine what makes a healthy immune system.

Most healthy adults – an estimated 80 percent to 95 percent – carry multiple herpes viruses, including one or both of those that cause mononucleosis.

The researchers, led by Dr. Herbert W. “Skip” Virgin, found that some of those viruses actually rev up the immune system, protecting against a food poisoning bacterium and a bacterium that causes the plague. Those findings suggests that herpes viruses are a crucial part of a well-tuned immune system, he said.

Immunity consequences

Virgin said scientists who are developing vaccines against the mononucleosis viruses should consider whether wiping out such infections could have negative consequences for the immune system. His group is now investigating how herpes viruses help protect against other infections.

He also is trying to determine whether herpes could get the immune system too keyed up “” causing autoimmune diseases, such as lupus, multiple sclerosis or rheumatoid arthritis.

Herpes viruses come in several varieties and can cause cold sores, chicken pox and genital herpes, as well as mononucleosis.

Some of the viruses produce only a mild fever or no symptoms. Other people get very sick or die from the infections.

“These are much more live-and-relate viruses than slash-and-burn viruses,” Virgin said.

Unlike flu or cold viruses, herpes viruses remain in the body for a long time, hiding out in nerve cells. The viruses rouse periodically from their dormant state.

When that happens, the body makes a hormone called interferon-gamma, which sends out a red-alert against bacterial invaders. When the body’s first line of defense, immune cells called macrophages, get the alarm, they turn into ruthless bacteria-killers.

Which herpes works?

The researchers infected mice with either the Herpes simplex virus 1, which causes cold sores, or the mouse-equivalent of two herpes viruses that cause mononucleosis in people.

Once the herpes viruses became dormant, the researchers gave the mice either a bacterium that causes plague; a bacterium called Listeria monocytogenes, which causes food poisoning; or West Nile virus.

Mice that had been infected with either of the mono viruses were able to fight off the two bacteria, but not West Nile virus.

Uninfected mice and mice infected with the cold sore virus “” a virus in the same class of herpes virus as genital herpes and chicken pox “” were not protected from either bacteria or the West Nile virus.

The study appears Thursday in the journal Nature.

Dr. Emil R. Unanue, a professor of pathology and immunology at Washington University who was not involved in the research, said the results suggest that “there is a certain reward” for having suffered a herpes virus infection.

“I still think that no infection is better than an infection,” he said.

St. Louis Post-Dispatch.

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Distributed by McClatchy-Tribune Information Services.

AP-NY-05-16-07 2155EDT