A fever is a temporary elevation of the body’s core temperature, a common response to an illness or infection. It is not an illness itself but a sign that the body is activating its defenses. This regulated increase in temperature is a natural response initiated by the immune system to help combat invading pathogens.
How the Body Creates a Fever
The generation of a fever is a controlled process orchestrated by the brain in response to specific triggers. It begins with the presence of substances called pyrogens, which can originate from outside the body (exogenous), such as toxins from bacteria, or from within the body (endogenous), like cytokines produced by immune cells. These pyrogens travel through the bloodstream to a region in the brain known as the hypothalamus, which functions as the body’s thermostat.
Once pyrogens reach the hypothalamus, they signal it to raise the body’s temperature set-point. The body then works to generate and conserve heat to match this new target. This is why a fever is often preceded by chills and shivering, as muscle contractions produce heat. Blood vessels in the skin also constrict to reduce heat loss and help raise the core temperature.
The Immune System’s Advantage at High Temperatures
This deliberately induced higher temperature is not just a symptom; it creates a strategic advantage for the immune system. The elevated heat directly interferes with the ability of many viruses and bacteria to replicate efficiently. These pathogens are often adapted to thrive at the body’s normal temperature, and the febrile environment slows their growth, giving the immune system more time to mount a defense.
Beyond hindering pathogens, the increased temperature enhances the performance of the body’s own immune cells. At febrile temperatures, certain types of immune cells, including T-cells and neutrophils, become more mobile and active. This allows them to travel more quickly to the site of an infection and carry out their functions with greater potency. The heat makes these defensive cells both faster and more effective.
The benefits extend to the molecular level as well. Key immune system proteins and enzymes function more efficiently in the heat. For instance, interferons, which are signaling proteins that help organize the antiviral response, show increased activity at higher temperatures. This boost in molecular efficiency means the communication and defensive actions within the immune system are amplified, contributing to a more robust fight against the infection.
When to Seek Medical Attention for a Fever
While a fever is a functional response, certain situations require medical evaluation. The threshold for concern varies significantly with age. For a newborn or infant under three months, any rectal temperature of 100.4°F (38°C) or higher warrants immediate medical attention, as their immune systems are not fully developed. For babies between three months and three years, a doctor should be contacted if a fever reaches 102.2°F (39°C) or higher.
For older children and adolescents, behavior is an important indicator alongside the thermometer reading. However, a fever that repeatedly rises above 104°F (40°C) or lasts for more than three days should prompt a call to a healthcare provider. In adults, a fever is not a cause for alarm unless it reaches 103°F (39.4°C) or persists for more than a few days. Medical advice is suggested if a fever at this level doesn’t improve.
It is important to look beyond the number on the thermometer. Certain accompanying signs should prompt a visit to a doctor regardless of the temperature. These include a severe headache, a stiff neck, an unusual skin rash, confusion, persistent vomiting, or difficulty breathing. These symptoms can indicate a more serious underlying condition that requires prompt treatment.