Well, the answer is not as simple as yes or no, unfortunately.
First of all, it depends on how sick you are. Secondly, it depends of the intensity of the physical activity.
When you are sick, your immune system is affected. A hard and intense workout creates stress on your body and even though a healthy individual can adapt to that stress, an already weakened person can get even more sick if they trains while feeling under the weather.
Does it mean you should stay in bed and watch Netflix all day until you start feeling better? No! Unless, you are too sick and tired to get out of bed, light physical activity can actually help you feel better. That includes anything that involves moving your body without creating any discomfort and being out of breath. For example, you can walk outside, do some yoga or do an easy bike ride. These non-strenuous activities actually help boost immunity and even though you might think it won’t have any impact on your current fitness level, some light physical activity is still better than not doing anything. Furthermore, some light physical activity while being sick will facilitate your transition to training once you start feeling better.
Strenuous activity is not recommended because, contrary to light physical activity, it depresses your immune system. A very long vigorous effort makes you more susceptible to infection. For instance, running a marathon can depress your immune system for up to 72 hours. This is why most people tend to get sick the following days of an intense event — because their body is more susceptible to infection.
So, stress from a strenuous physical activity can add up to the stress caused by your sickness, but other stress in your life, such as stress caused by your work, your family responsibilities and emotional stress, can also affect negatively your recovery. Indeed, according to the New England Journal of Medicine, psychological stress could influence immune function through autonomic nerves innervating lymphoid tissue or hormone-mediated alteration of immune cells. You should therefore try to limit the stress in your life. For instance, you could take a sick day from work and rest at home. You could also limit your consumption of social media, especially at night, since it has been shown to cause stress and affect sleep. You should also make sure you get as much sleep as possible, by either going to bed earlier or taking a nap during the day if you have the opportunity.
Finally, you should eat healthy food and drink plenty of water. Consumption of vitamin C can decrease the severity and duration of colds, but it is not necessary to buy over-the-counter supplements. Indeed, the recommended intake of 0.2g or more per day, can be easily obtained from the diet. The greatest benefit of vitamin C intake has been showed when intake was high within 24 hours of the first symptoms of a cold. Citrus fruits, cantaloupe, kiwi fruit, mango, pineapple and berries are great sources of vitamin C.
So in conclusion, you can still do some light exercise when you are sick, but you shouldn’t do any strenuous activity that could depress even more your immune system. Also, reduce your stress in your life, eat well, drink plenty of water and sleep well.