Influenza patients usually recover from fever in 5-7 days. Elderly people, pregnant women, infants and children have a longer duration of illness due to weaker immune systems. Influenza is caused by influenza virus infection. Patients may have fever, cough, phlegm, sore throat, runny nose, sneezing, headache, and fatigue. Influenza is a self-limiting disease and the body can clear the influenza virus on its own when the immune function is good, so that the body temperature returns to normal and patients usually have fever for 5-7 days. In special groups, such as the elderly, pregnant women or infants, the fever lasts longer, up to 10-20 days, because the immune system is weaker. If the flu leads to secondary pneumonia, bronchitis, pleurisy, or even systemic sepsis, fever symptoms can last longer and require hospitalization. In addition, influenza patients, especially those with weakened immunity, should take antiviral drugs, such as oseltamivir, in a timely manner, preferably within 48 hours of onset, to ensure efficacy as much as possible.