A fever is defined as a body temperature of 100.4°F (38°C) or higher. This temporary rise in temperature is a physiological response orchestrated by the immune system to fight invading pathogens. Common causes include viral infections, such as the flu or common cold, and bacterial infections, like strep throat or urinary tract infections. Fevers are typically self-limited, resolving as the body successfully fights off the infection within a few days. Determining when to seek medical evaluation depends heavily on the person’s age, overall health status, and accompanying symptoms.
Fevers in Infants (Ages 0–3 Months)
Advice for infants aged 0 to 3 months is much stricter than for any other age group due to their immature immune systems. Any fever in this population is considered a medical emergency until a serious cause is ruled out. A rectal temperature of 100.4°F (38°C) or higher warrants immediate medical evaluation.
This zero-tolerance approach exists because serious bacterial infections, such as sepsis or meningitis, can progress rapidly in this age bracket. The risk of a bloodstream infection is higher than in older children, as the infant’s body is often unable to localize the infection. Even if the baby appears well, the temperature threshold alone requires urgent attention for immediate testing and potential treatment.
A healthcare provider will typically perform a full workup, which may include blood, urine, and spinal fluid analysis, to determine the source of the fever. Delaying treatment for a serious infection in a newborn can have severe consequences. Parents should go directly to the emergency room or contact their pediatrician immediately if their infant registers this temperature.
Duration Guidelines for Older Children and Teens
For children over three months old up to the teenage years, the duration of the fever plays a larger role in determining when to seek care. If a child between three months and two years old has a fever, contact a doctor if the temperature persists for more than 24 hours. This shorter timeframe helps catch potential issues early in this younger age group.
For children older than two years, contact a healthcare provider if the fever lasts longer than 72 hours, or three days. Most fevers caused by common viral illnesses resolve within two to three days, so persistence beyond this point may indicate a more complex issue. Mild symptoms, like a runny nose or occasional cough, are acceptable to monitor at home within these time frames, provided the child is drinking fluids and remains alert.
A fever that reaches 104°F (40°C) or higher requires a call to the doctor regardless of duration. Medical advice should also be sought promptly if the fever initially improves but then returns, or if the child becomes sicker despite the fever breaking. The child’s overall appearance and behavior, including their activity level and hydration, are often more important indicators than the temperature reading alone.
Duration Guidelines for Adults
Adults typically have more flexible guidelines for fever duration, as their mature immune systems are better equipped to manage common infections. A healthy adult should seek medical advice if a fever persists for more than three to five days. This duration allows sufficient time to combat a typical viral illness.
If an adult’s fever is 103°F (39.4°C) or higher, they should contact a doctor sooner, even if it has not been three days. Individuals with underlying chronic conditions, such as diabetes, heart disease, or a compromised immune system, should seek care much sooner. For these high-risk patients, even a mild fever can signal a serious problem and warrants prompt evaluation.
Adults should also seek care if the fever does not respond to over-the-counter fever-reducing medications like acetaminophen or ibuprofen. A fever that worsens, returns after initial improvement, or is accompanied by signs of dehydration should also trigger a call to a health professional.
Warning Signs Requiring Immediate Care
Certain acute symptoms override all duration and temperature guidelines, demanding immediate emergency medical attention. These symptoms indicate potentially life-threatening conditions that require rapid intervention regardless of the patient’s age or how long the fever has lasted.
A severe headache combined with a stiff neck is a warning sign of meningitis. Extreme lethargy, confusion, or difficulty waking up also require immediate care, as these indicate potential neurological involvement. Any rash that does not fade or blanch when pressed, particularly if it appears as purple spots, suggests a severe bloodstream infection and is a medical emergency.
Other severe symptoms include difficulty breathing, severe chest pain, or rapid, shallow breathing. Persistent vomiting or an inability to keep fluids down can quickly lead to severe dehydration and is a cause for immediate concern. The sudden onset of a seizure, severe abdominal pain, or a bulging soft spot on an infant’s head necessitate an emergency room visit without delay.