It is impossible to generalize about the worst days of fever caused by viral infections and is closely related to the patient’s resistance and the type of virus. Fever is a protective response of the body against pathogens, and viral infections can cause fever, i.e. viral fever. Generally, common viral infections cause fever for about 3-5 days, without special treatment, and are self-limiting, usually the most serious on the third day. Influenza virus has relatively severe symptoms and the fever lasts for a relatively long time, which may last for about 1-2 weeks. Some specific viral infections, such as EBV infection, have fevers that can last for several weeks. In addition, once a patient develops a bacterial infection, such as septicemia or gram-negative bacillus infection, on top of the viral infection, the fever will be prolonged accordingly, and may even last for more than 2 weeks. In addition, patients should take care to rest in bed during the fever period, and drink more water to improve body metabolism.