The immune response is a potent protection system against infective agents (such as the flu). The thing is, as it fights against the flu, it affects the body cells as well. By “affects” I mean damages and kills them. As a result, the pain in the muscles and joints kick in (within the first 2 or 3 days of a disease and is present for 4 to 5 days).
Fatigue is an unspecific symptom, but is the first sign of flu. Some people find it is so severe they can not get out of the bed for days. Because of different hormonal settings, women tend to handle flu symptoms better. This is possibly the reason for the popular term ‘man flu’.