How long an adult should not have a high fever of 39°C is determined by the specific type of fever, the condition of the primary disease, and the presence of comorbidities. The normal human body temperature is 36℃-37℃, and a body temperature of 38.1℃-39℃ is considered moderate fever. Generally, after effective treatment, adults with moderate fever can be cured in about 3-5 days, or have different degrees of improvement. When there are obvious lesions in the body organs, fever can last for more than 1 week or even longer, and patients can develop acute and critical illnesses, or irreversible brain cell damage and multiple organ failure can occur. Adults with fever over 39℃ can take appropriate medication to reduce the fever. If the body temperature rises repeatedly, you should go to the hospital infection department, emergency department or fever clinic in time, follow the medical advice for examination and receive treatment such as fever reduction and anti-infection.