The Running Doctor: Colds need rest
The athlete with a cold needs rest, not exercise. One should not run or exercise vigorously with a cold, flu, or any viral infection that causes a fever. Although only certain organ systems produce symptoms, the entire body is affected.
It is very important to keep a daily record of your morning pulse when you begin training for any sport or type of exercise program.
While still lying in bed, take your pulse a few minutes after awakening in the morning. As your training progresses, your pulse will gradually become slower. After two or three months, it will plateau. After this point, if your rate is ten or more beats higher one morning, it means you have not recovered from your previous day's run, race or stress. It is then advisable to take at least one day off until the pulse returns to normal.
The cold is an early warning sign of exhaustion resulting in a, breakdown of the body's defense system. Influenza may then gain its entry to the body systems starting with a sore throat and then possibly progressing to a cough. This virus in some way has the ability to get into the cells, reproducing itself, attacking the outer layer of the cell, killing it and moving on to attack another cell.
The utmost caution should be taken when you return to exercise, because it's very difficult to know how much fatigue has built up during the actual lay-off from your sport. This body fatigue often leads to the overuse syndrome.