A fever is a temporary elevation in the body’s temperature, defined by a reading of 100.4°F (38°C) or higher. This rise is not an illness in itself but a sign that the immune system is fighting off an infection, such as a virus or bacteria. The increased internal temperature makes the environment less hospitable for pathogens while boosting the body’s defensive response. Knowing the expected duration and recognizing specific symptoms is important for determining when professional medical evaluation is necessary. A fever’s significance changes dramatically based on a person’s age and accompanying physical signs.
Understanding Typical Fever Duration
The duration of a fever is strongly influenced by its underlying cause, most commonly an acute viral infection. For the majority of healthy adults and children, a fever caused by a common cold or flu-like virus typically resolves within 24 to 72 hours. This short-term fever indicates that the body is successfully neutralizing the infectious agent.
While many fevers subside quickly, some can persist for a longer period. Viral fevers, such as those associated with respiratory syncytial virus (RSV) or influenza, may last up to five to seven days before the temperature returns to the normal range.
A fever that lasts longer than 72 hours, or three days, generally warrants seeking evaluation, even if the temperature is not extremely high. This prolonged duration can signal a bacterial infection that requires specific treatment, or it may indicate a more complex underlying condition. Fevers that come and go over a period exceeding two weeks are considered chronic and require a thorough diagnostic workup.
Age-Specific Guidelines and Temperature Thresholds
The urgency of medical attention is highly dependent on the patient’s age. For newborns and infants up to three months old, any temperature of 100.4°F (38°C) or higher, measured rectally, is considered a medical emergency. The immature immune systems of these young infants mean that even a moderate fever can be the only initial sign of a serious bacterial infection. Immediate evaluation in an emergency setting is required for this age group, regardless of how alert or well the baby appears.
For older infants and children between three months and two years, the threshold for concern incorporates the child’s behavior. A fever above 102°F (38.9°C) that persists for more than 24 hours, or any fever accompanied by increased irritability, lethargy, or refusal to drink fluids, necessitates contacting a healthcare provider. If the temperature climbs to 104°F (40°C) or higher, medical advice should be sought promptly.
In healthy adults, the body is generally capable of tolerating temperature elevations. Adults should seek medical attention if the fever reaches 103°F (39.4°C) or higher, or if a lower-grade fever persists for more than three days without improvement. The focus remains on the temperature level and the duration of the elevation.
Warning Signs Requiring Immediate Medical Attention
Certain symptoms accompanying a fever are considered red flags that necessitate immediate medical attention, overriding concerns about temperature level or duration. Neurological changes are especially concerning and include severe headache, sudden confusion, or difficulty arousing the patient. A stiff neck, particularly when combined with a sensitivity to bright light, can indicate irritation of the membranes surrounding the brain and spinal cord.
Acute respiratory distress should be treated as an emergency, signaled by difficulty breathing, rapid or shallow breaths, or persistent chest pain. Repeated or persistent vomiting and the inability to keep down liquids are serious signs of potential dehydration, which can rapidly worsen the patient’s condition. For children, signs of severe dehydration include sunken eyes, a dry mouth, and crying without tears.
The sudden appearance of an unexplained rash, especially one that does not blanch or fade when pressed, also warrants immediate medical evaluation. Any instance of a seizure or convulsion linked to the fever is a critical event that requires emergency care.