Committed exercisers should try to continue to exercise during this COVID-19 pandemic, but they should realize that both too much exercise and exercising while sick increase risk for medical complications, such as irregular heartbeats, and death (Brit J of Sports Med, Sep 4, 2009;43(9):722-725). We have strong evidence that even a single workout improves your immune system (Med Sci Sports Exerc, 2004;36(8):1321–7), which will help you fight off COVID-19, as long as you don’t exercise too much (Brain Behav Immun, Sept 2005;19(5):377-80), or increase your risk for exposure to a COVID-19 infection by going to a gym used by other people, exercising too close to people, breathing air breathed out by infected people, or touching equipment used by others. Remember to wash your hands before and after you exercise, don’t exercise near anyone else (particularly one who is sneezing or coughing), and rub an alcohol wipe over exercise equipment before and after use. Skip your workout completely if you feel sick.

Don’t Exercise If You Feel Sick
COVID-19 can be a very dangerous virus that starts in your mouth and nose and then can travel down into your lungs with potentially severe consequences, so it is not a good idea to exercise if you have symptoms of a COVID-19 infection, particularly any evidence of lung involvement such as coughing, wheezing or shortness of breath. With this virus and most other upper respiratory infections, you may be able to exercise when you have a stuffy nose or sneezing, but it is very risky to exercise when the infection has passed down into your lungs. If you try to exercise and don’t feel better in 5 to 10 minutes, you should take the day off. Whatever you do, when you are sick, you should not exercise intensely. Take off if you have any of the following: feeling sick or weak, coughing, being short of breath, having muscle aches and pains, having a fever higher than 98.6°F or 37°C, nausea, diarrhea, vomiting or stomach cramping.

Evidence that Exercise Benefits Your Immune System
As long as you are healthy and are not experiencing symptoms of a possible infection, any form of exercise will be beneficial.
• Working out regularly has been shown to decrease the risk of many chronic diseases (Compr Physiol. Apr, 2012;2(2):1143–1211) such as diabetes (Diabetes Care, Dec 2010;33(12)) or cancer (Exerc Immunol Rev, 2013;19:120-43).
• Elite marathon runners report fewer days lost from work than non-exercisers (J Sports Sci Med, 2014 Dec 1;13(4):929-33).
• A study of 25,000 Chinese who died during the 1998 Hong Kong flu epidemic found that those who exercised moderately three times a week were far less likely to die (Exercise Immunology Review, December 2019;26:8-22; PLoS ONE, 2008;3(5):e2108).
• Rats that ran moderately on a treadmill for 30 minutes a day for several weeks were far more likely than non-exercising rats to survive rat influenza (Brain Behav Immun, Sept 2005;19(5):377-80).
• Exercise-trained mice injected with germs immediately after running on a treadmill are less likely to become sick and die than those who do not exercise, because vigorous exercise causes immune cells to surround and attack invading germs, rather than having their immune cells scattered throughout their bodies and not attacking the invading germ (Eur J Physiol, May 20, 2020;472:235–244).
• Mice that exercised regularly for three months prior to an induced infection suffered significantly less severity and duration of illness and lower blood virus levels than non-exercising mice (J of Infect Dis, Oct 2009;200(9):1434–1442). The same results occurred for both obese and normal weight mice.

Don’t Exercise Too Much
A large increase in exercise intensity or duration can reduce your immunity (Exerc Sport Sci Rev, Oct 2009;37(4):157–164) and increase risk for severity and death when you have respiratory disease (Scand J Med Sci Sports, 2006;16(4):287–93). Exercising improves your immune system, but too much exercise increases risk for infections. For example, over a five-month period, people who did not exercise suffered twice the rate of upper respiratory infection suffered by recreational exercisers, but elite athletes training near their maximum suffered the most upper respiratory infections of all — three times the rate of upper respiratory infections that recreational exercisers did (Med Sci Sports Exerc, Apr 2007;39(4):577-86). You may be stressing your immune system when you take workouts that:
• last longer than 90 minutes, especially without taking food and fluids,
• are of very high intensity, greater than 85 percent of your maximum effort, and
• fail to allow adequate recovery periods between intense workouts (Journal of Applied Physiology, Aug 1, 2007;103(2)).

How Fast Will I Lose My Fitness If I Don’t Exercise?
Within two weeks after healthy, young people reduced their walking steps from more than 10,000 steps a day to fewer than 2,000 steps, they had higher blood sugar levels and cholesterol levels, and lower insulin sensitivity (Diabetologia, 2018 Jun;61(6):1282-1294). Four weeks after marathon runners reduced their mileage after a marathon, they had lower blood volume and reduced ability of their hearts to pump blood, which markedly slowed their ability to run fast on a treadmill (J of Appl Physio, April 1, 2018;124(4)). On the other hand, after just six weeks of running up and down stairs for 20 seconds three times a day, people increased their aerobic fitness by about five percent (Appl Physiol Nutr and Metab, Jan 16, 2019;44(6):681-684).

My Recommendations
Most healthy people, particularly those who exercise regularly, should try to exercise during stay-at-home periods of the current pandemic. You can exercise in your home or yard, and most of these orders allow for outdoor exercise as long as you maintain the required distances from other exercisers. You should not:
• exercise within six feet of other people
• exercise in a gym used by other people
• share exercise equipment with people other than those in your household
• use any exercise equipment without first cleaning it off with alcohol wipes
• share drinks or eating utensils
• overdrink fluids (it is not possible to “flush out toxins”)
• exercise to exhaustion
• exercise when you have flu-like or other symptoms of illness (described above)