Raising the heat helps animals battle germs—what humans can learn from fevers

The Science Behind Fevers and How Animals Use Heat to Fight Infections

Fevers are a natural response that the body uses to combat infections, but many people don’t fully understand their purpose. While it’s commonly believed that pathogens like SARS-CoV-2 or the flu directly cause fevers, the reality is more complex. According to Phil Starks and Harry Bernheim, biology professors at Tufts University, pathogens contribute to fevers indirectly. When the immune system detects harmful microbes, it triggers a rise in internal temperature to create an environment that’s hostile to these invaders. This process helps suppress the spread of pathogens and is essentially the body's way of fighting back.

This biological mechanism isn't unique to humans. Many animals, including lizards, fish, and insects, also use fever as a defense strategy when they fall ill. Even so-called "simple creatures" have evolved this ability, demonstrating that fever is a widespread and effective survival tactic across species.

How the Body Responds to Infection

When a person catches a virus, the immune system releases molecules called pyrogens, which signal the brain's hypothalamus to raise the body’s set point temperature. Normally, the body maintains a temperature around 98.6 degrees Fahrenheit (37 degrees Celsius), but during a fever, this can increase to 100.4 to 104 F (38 to 40 C). As the body works to reach this new temperature, muscles contract, causing shivers, and blood vessels constrict to retain heat. People often feel cold until the temperature stabilizes, prompting them to add layers or seek warmth. Once the infection subsides, pyrogens decrease, and the hypothalamus resets the temperature. At this point, the body begins to cool down through sweating and dilated blood vessels, and the person starts to feel better.

Fevers in Mammals, Reptiles, Fish, and Insects

Humans are not alone in their ability to generate fevers. All mammals, including dogs, exhibit similar behaviors when they have a fever—losing appetite, becoming lethargic, and shivering. This response is also seen in cold-blooded animals, which rely on their environment for warmth. For example, lizards will move to warmer areas when sick, and if prevented from doing so, their survival rates drop. Zebrafish swim to warmer waters during infections, and even a small temperature increase can improve gene expression and antiviral responses. Naked mole rats, despite being cold-blooded, also generate fevers when infected.

Insects, too, show remarkable responses. Desert locusts elevate their body temperature in a dose-dependent manner, with higher pathogen exposure leading to higher temperatures. Honeybees are among the most sophisticated in this regard. They regulate hive temperatures precisely, using flight muscles to warm the space and fanning wings to cool it. If larvae are exposed to fungal spores, the colony raises the temperature to protect the next generation. Once the threat passes, the bees return to normal hive conditions.

When to Seek Medical Attention

While fevers are a beneficial response, they should not be ignored. If a fever persists for more than 24 to 36 hours, it may indicate a more serious condition and requires medical attention. Adults should see a doctor if a fever exceeds 103 F (39.4 C), while children should be evaluated if the temperature reaches 102 F (38.9 C). Infants under three months old should be checked if their temperature hits 100.4 F (38 C).

Despite the instinct to reduce fevers using medications like aspirin or by cooling the body, mild to moderate fevers often help the body fight infection. Reducing a fever too soon may interfere with the immune response and prolong illness.

A Historical Perspective on Fever Treatments

The idea of using fever to combat disease is not new. In the early 20th century, Austrian physician Julius Wagner-Jauregg pioneered a method called malariotherapy, where he infected syphilis patients with malaria to induce high fevers that killed the bacteria causing syphilis. Although risky, this approach was effective enough to earn him the Nobel Prize in 1927. However, it was eventually replaced by penicillin due to its dangers and side effects.

Convergent Evolution and the Power of Fever

The fact that so many diverse species have developed similar fever responses highlights the concept of convergent evolution. Despite different evolutionary paths, organisms facing the same challenge—infection—have arrived at the same solution: fever. This suggests that the body's ability to raise its temperature is a powerful and essential adaptation for survival.

Understanding how fever affects the immune response remains an area of ongoing research. However, one thing is clear: fever is a vital tool in the body's defense against infection.