The sudden onset of fever in the elderly is usually caused by various infectious diseases. Common infectious diseases include upper respiratory tract infections, lung infections, urinary tract infections, skin and soft tissue infections, blood-borne infections, and central nervous system infections. When the pathogens of these diseases invade the body, they release a large amount of toxins, which can disrupt the body’s thermoregulatory center and cause fever symptoms. In addition, dysfunction of the thermoregulatory center is also a common cause of sudden fever in the elderly, and its called central fever, including heat stroke and traumatic brain injury. There are also non-infectious diseases that can cause sudden fever in the elderly, including leukemia, lymphoma, rheumatoid arthritis, and systemic lupus erythematosus.