Like all good things in life, science says you can overdo exercise. Pushing yourself too hard for too long can actually put you at higher risk of infection—but you have to go pretty far past that “acute” level of training to experience negative side effects.

For example, most studies that found that extreme exercise can increase risk of illness examined marathon runners, like Nieman's 1990 study published in the Journal of Sports Medicine and Physical Fitness. But Nieman says this negative effect can come into play if you're running at a high intensity for at least a half-marathon distance or cycling or swimming at a tough pace for about 90 minutes. Any of these longer, more intense activities can cause stress on the body that could lead to lowered immune function. “You put yourself in a stressful state, so your immune system reflects that and leads to dysfunction that can last anywhere from a few hours to a few days,” Nieman says. Basically, high-intensity activity for more than an hour might not be the best idea right now if you’re really focusing on keeping your immune system in top shape.

Jajtner agrees that spending weeks incorporating super high-intensity workouts into your schedule could backfire. "If you’re looking at exercise in a chronic sense, there’s something we refer to as the J curve—over time, as you continue to exercise at a moderate intensity for weeks to months, your risk for infection will decrease," says Jajtner. "But if you do excessive and intense exercise, you increase risk of infection."

Exactly how long and how hard you can push yourself before you reach that excessive and intense level of exercise ultimately comes down to how well you’re trained, but you might want to focus on maintenance rather than intensity in these pandemic times. “Moderate intensity is the best route right now, but maintaining that activity, in some form or fashion, is going to be key,” Jajtner adds.