Influenza, commonly known as the flu, is a highly contagious respiratory illness caused by a virus that infects the nose, throat, and lungs. When the body encounters the influenza virus, it launches a noticeable defense mechanism characterized by a sudden change in body temperature. The shift from feeling cold to hot, known as fever, is not merely a symptom of the infection. This thermal response is an adaptive maneuver by the immune system to create an environment hostile to the virus.
The Science of Fever Generation
The process of raising the body’s internal temperature begins when the immune system detects viral material. Immune cells, such as macrophages, respond by releasing signaling molecules called pyrogens, which travel through the bloodstream. These pyrogens communicate the presence of an infection to the brain’s thermoregulatory center.
The hypothalamus, a small structure deep within the brain, acts as the body’s thermostat, maintaining a precise temperature set point, normally around 37°C (98.6°F). Pyrogens trigger the production of prostaglandin E2 (PGE2) within the hypothalamus, which resets the thermostat to a higher temperature. The body then perceives its current temperature as too low relative to this new set point.
This disparity immediately activates heat-generating and heat-conserving responses. The body begins to shiver, causing rapid muscle contractions that generate internal heat, which is why a person may feel chills even when their temperature is rising. Simultaneously, blood vessels near the skin’s surface constrict (vasoconstriction) to minimize heat loss. These actions continue until the core body temperature reaches the hypothalamus’s new setting.
Fever’s Role in Immune Defense
The elevated temperature achieved during a fever serves several biological purposes that directly support the immune response against the influenza virus. One primary benefit is the direct inhibition of viral replication. Temperatures in the febrile range, such as 39°C or 40°C (102.2°F or 104°F), can interfere with the influenza virus’s machinery by affecting the function of viral RNA polymerase. The heat also appears to increase the pH inside endosomes, small compartments the virus uses to enter the host cell, thus disrupting the infection process.
Higher temperatures significantly enhance the efficiency of the body’s immune cells. Fever promotes the migration of lymphocytes, a type of white blood cell that includes T-cells, toward the sites of infection. This movement is facilitated by the heat-induced expression of proteins like heat shock protein 90 (Hsp90), which helps immune cells adhere to blood vessel walls and move into infected tissues quickly.
The temporary thermal increase activates certain genes and proteins that help regulate the immune system’s response. By speeding up these cellular mechanisms, fever ensures the defense system can mount a timely attack against the pathogen. The fever response is not an accidental side effect of illness but a regulated strategy designed to make the body a less hospitable environment for the virus while boosting the capabilities of the immune forces.
Strategies for Managing Temperature Changes
While fever is a beneficial defense mechanism, managing the associated physical discomfort and recognizing warning signs are important aspects of flu recovery. For many adults, a temperature at or below 38.9°C (102°F) can be managed with simple home care, focusing on comfort rather than aggressive temperature reduction. The goal of treating mild fever is to alleviate symptoms like headache and muscle aches, allowing for better rest.
Non-pharmacological strategies center on maintaining hydration and regulating external temperature. Drinking plenty of clear fluids, such as water or broth, is important because fever increases the risk of dehydration due to fluid loss through sweating and increased metabolism. When chills are present, using a light blanket helps the body reach the new set point without excessive shivering. Removing layers when sweating begins aids in natural cooling.
Antipyretic medications, such as acetaminophen or ibuprofen, work by blocking the synthesis of prostaglandin E2 in the hypothalamus, lowering the set point back toward the normal range. These medicines can provide relief from discomfort, but they should be used according to recommended dosages. Avoid combining multiple medications that contain the same active ingredient, which can lead to accidental overdose.
Recognizing when a fever requires medical attention is necessary. Seek professional advice if a fever rises above 40°C (104°F) in adults, or if it is accompanied by concerning symptoms like severe headache, neck stiffness, difficulty breathing, or confusion. For infants younger than three months, any rectal temperature of 38°C (100.4°F) or higher warrants immediate medical evaluation. Rest and sleep remain the most straightforward ways to allow the body to focus its energy on fighting the infection.