Dear Welcome to HCM We understand your concerns Why fever and tiredness? Body is infected with some foreign bodies and our body is supposed to fight hard to maintain health. Body spent every energy it have to over power the foreign bodies which caused infection or disease. This causes body heat to rise and you are feeling tired. Body even don;t allows to eat food (low appetite or vomiting)because body knows that it needs to spend energy to digest the food. Therefore, you need proper and full rest for at least three days if you are affected with flu. allow the body to do job and regain health. Get well soon.