Infants with high fever of 40℃ are recommended to take fever-reducing drugs, such as Tylenol or Merlin, immediately. Parents should take their babies to the pediatrician of the hospital in time to avoid delaying the condition. Exactly how long it lasts is dangerous to occur needs to be determined on a case-by-case basis. Usually, the body will take away some of the water when it dissipates heat. If the baby continues to have a high fever of 40℃, it may lead to a significant loss of water in the body, and if it is not replenished in time, it is likely that dehydration will occur within minutes or tens of minutes, which in turn may be dangerous. And infants with a high fever of 40 ℃ lasting 30 minutes may lead to edema and necrosis of brain cells, which may cause some damage to the central system. In addition, the baby’s heart rate and respiratory rate may increase significantly during a sustained high fever, and prolonged tachycardia may induce cardiac insufficiency, usually after a few hours. In addition, parents should also be aware that infants with persistent high fever of 40°C must pay attention to hydration while seeking timely medical attention to avoid dehydration and other conditions that may aggravate the condition.