Running 10 miles a day, every day, in rain, snow and cold could make you a hero to the running club, but it may not impress a germ. One expert says tough workouts may lower resistance to disease.

Intense aerobic workouts may increase the body's production of hormones that depress the immune system, said David C. Nieman, an associate professor in health, leisure and exercise science at Appalachian State University in North Carolina.On the other hand, moderate aerobic activities such as walking can increase the activity of cells that feast on bacteria and viruses, making you better able to resist disease, Nieman said.

While Nieman was at Loma Linda University in California, he and his colleagues looked at how many of the 2,300 runners who had completed the Los Angeles Marathon in 1987 came down with a cold or the flu.

More than 13 percent were sick within a week, compared with 2 percent of those who trained but didn't compete, Nieman said.

And it's not simply the stress of competition, the researcher said. In the two months before the race, approximately 40 percent got sick. Those who trained more than 60 miles a week had twice the risk of those who trained less than 20 miles a week, he said.

The cause could be hormonal, Nieman said. Ten experienced runners who worked out for three hours at their fastest marathon pace had higher levels of the hormone cortisol - close to 60 percent above their pretest levels, he said.

Cortisol is valuable for people who do long, hard exercise, because it fights inflammation. But it also depresses the activity of natural killer cells, which destroy virus-infected cells, Nieman said. His work with the 10 experienced runners, while preliminary, indicated that natural killer cell activity fell by more than 30 percent for almost six hours.

He advises serious athletes to do no more than two workouts a week at their hardest level, to give the immune system time to recover.

However, exercisers who don't work out so hard don't risk weakening their immune systems as they try to improve their fitness.

Instead, regular, moderate exercise may perk up your immune system, Nieman said. He bases this on a study of 50 women, 12 of whom were started on a program of 45-minute brisk walks, five days a week.

The walkers increased their natural killer cell activity, Nieman said. "It appears walking helps to pull the cells out of your spleen into circulation."