Do you know people that like to exercise when they don't feel well? There is the old saying about "sweating out a cold." Is that saying scientifically accurate?
No, the statement is just folk wisdom, not science.
If someone just has a runny nose or sore throat, it may be alright to do some moderate exercise for a half hour or so. But if they are achey and have a fever, then it's better to rest.
You may have also heard that regular physical activity can strengthen your immune system and help prevent catching cold and other illnesses. So people think that even if they're already sick, doing something physical may help.
The answer is that it might, but it depends on how sick they are. In some cases, if someone has a fever, working out can tax their body and make the illness worse.
People should rest to regain strength. And when they're coming out of a really bad cold or fever, it's best to ease back into exercise instead of jumping right back in.