Most doctors allow their patients to exercise when they have a cold, as long as they don't have a fever and their muscles don't hurt when they exercise. However, it's probably better to stop exercising altogether when you have an infection. You risk injury if you exercise when your muscles hurt at rest or when you start to exercise. When muscles are damaged, they release enzymes from their cells into the bloodstream and they fill with blood from broken blood vessels. One study reported markedly increased muscle damage during relatively minor exercise during an infection, with blood tests demonstrating increases in muscle enzymes and ultrasound tests demonstrating hemorrhage into the muscles.
You also should not exercise when you have a high fever. When you exercise, your heart has to pump blood to your muscles to supply them with oxygen. It also has to pump blood from your muscles to your skin where the heat is dissipated. When you have a fever, your heart has to work extra hard to get rid of extra heat. Furthermore, some viruses that infect your nose and throat can also infect your heart muscle. The combination of the extra work and an infected heart muscle could cause irregular heart beats. You won't lose much conditioning unless you take off for more than a week.
Do you think you will get better faster by staying in bed for a day or two or by going about your normal daily activities? A huge review of the medical literature from 1966 to 1999, published in the British medical journal Lancet, shows that there is no evidence that bed rest helps you to heal faster from any medical condition. They found only 39 studies testing whether bed rest benefitted any medical condition. Twenty-four studies showed that bed rest was of little or no benefit in preventing side effects of medical procedures such as spinal anaesthesia, spinal fluid withdrawal, and multiple x ray procedures. Fifteen studies showed no benefit in treating medical conditions such as low back pain, spontaneous labor, high blood pressure during pregnancy, uncomplicated heart attacks, rheumatoid arthritis or infectious hepatitis.
During World War II, American soldiers were drafted and sent to do their basic training at the Great Lakes Naval Training Center before they were sent to fight in Europe. A major epidemic of flu occurred affecting almost all the troops. Half of the soldiers were kept in bed, while the other half stayed in the vigorous exercise of basic training. Both groups required the same amount of time to recover, although those forced to undergo the rigorous demands of preparing for war complained more.
The next time you feel sick, stay in bed if you like, but it probably won't help you heal faster.