When you do come down with a cold, use the "neck rule," recommended by many sports physiologists to determine whether or not it's OK to exert yourself. If symptoms occur above your neck (runny nose, sneezing, scratchy throat, but no fever), working out is just fine; if your troubles lie below the neck (coughing, nausea, achy joints), you need rest. Researchers at Ball State University in Muncie, Indiana, recently infected 45 students with a rhinovirus that causes colds and their associated symptoms, such as runny noses and sore throats. After the infected students and ten healthy counterparts ran on a treadmill until exhaustion, researchers found that the VO₂ max (or oxygen uptake) of the infected students was not impaired. Surprisingly, the sick students reported that exercising didn't feel any more strenuous than usual.

In a separate study, those same researchers at Ball State compared the symptoms and duration of the colds of 34 people who worked out every day for 40 minutes at 70 percent of maximum heart rate with those of 16 people who had colds but didn't work out. The verdict? "There's no difference to the duration or severity of your cold if you work out while you have a cold," says Thomas Weidner, the study's lead researcher. In fact, if you've just got a mild cold, working out may even make you feel better. Exercise releases adrenaline, a natural decongestant, which may explain why a run or other activity can help clear nasal passages. "There's a psychological tendency to not want to work out when you've got a cold," says Weidner. "But once our subjects started working out, they reported that they didn't feel any worse." Now if only they'd stop sneezing on the free weights.

EASY DOES IT

Here's the tricky part about the neck rule: It's easy to convince yourself that certain symptoms, like a cough that comes and goes, don't count. Running, cross-country skiing, playing hockey, or pursuing other aerobic activities in cold weather will make you cough even when you're not ill (the cold air stimulates mucus production in your throat). However, if you have a persistent or hacking cough, or if your coughing brings up mucus, you could well have an infection in your airways, and your lung capacity may be diminished. Avoid heavy aerobic exercise—it can spread the infection. "There's no reason you can't do some light toning and weight work if you're coughing too much to run," says Maharam. "But stick to that."

Keep your activity to half-hour weight sessions or nonstrenuous yoga classes, drink copious amounts of water to make sure your mucus membranes stay well hydrated, and take breaks to let your heart rate—and your breathing—return to a baseline level.

In the case of a fever, let your temperature be your guide. When your body temperature is above normal, fluids may be diminished by 5 to 10 percent. A fever indicates that your body is waging a battle against a virus, so the last thing any sane individual wants to do is sap his forces. "If your fever is 100.5 or above, you should not work out," Maharam intones. "If you're taking any kind of pain reliever to keep it down, you should not work out."

Unlike your run-of-the-mill cold virus, flu viruses often produce a much more serious illness, with cough, sore throat, runny or stuffy nose, headache, muscle aches, extreme fatigue, and most significant, fever. In an average year, flu is an accessory to 20,000 deaths nationwide, most of whom are elderly or suffering from autoimmune deficiencies. But be warned: For the young and healthy, vigorous exercise with the flu may cause a virus to invade the heart or the sac that surrounds it, which can be fatal. That said, don't cut back on nourishment because you can't exercise—your body needs the calories to fight the illness.

If you don't have a fever but do have nausea, vomiting, or diarrhea—all of which can result in dehydration—you should also expect to be out of the game for a spell. The level of electrolytes in your body changes when your fluids are reduced, putting you at risk of muscle cramps, spasms, and in severe cases, heart arrhythmias. Rehydrating can take several days, so it's best to avoid running, Spinning, or other sweat-inducing exercises while your stomach is staging a coup on your well-being. This also includes swimming: You may not realize it, but while you're doing the backstroke, you're leaking electrolytes all over the pool.

A GOOD RECOVERY

If you think you're ready to hit the trail once your temperature is below 100.5 and you've thrown out the Robitussin, you're half right. If you've had a fever, resume your regular workout at about half your normal pace once symptoms have cleared. "For every day you were sick, take three days to return to your previous intensity," advises Maharam. Consequently, a two-day flu warrants six days of light workouts. If you typically run five miles four times a week, try 2.5 miles on your first day out and gradually work up to running your full, healthy pace and distance on day seven. If you typically do a full-body weight-lifting routine of three sets of ten to 12 repetitions, try instead one to two sets at 50 percent of your training weight.

Now is also a good time to drink sports drinks or watered-down apple juice to return your electrolytes to their normal level, along with plenty of water to rehydrate. Of course, quit exercising if you're dizzy or nauseated, start sweating heavily, or feel so weak and wobbly that you can't maintain your form. Your goal should be to get back to where you were as quickly as possible, but it's important to self-monitor to make sure you don't do too much too soon. "It's classic sports medicine," says Maharam. "Use your best judgment."

Tish Hamilton is a marathoner and former managing editor of Outside.

Photo: Craig Cameron Olsen