Why do people get fevers when we get sick?
It’s a common misconception that pathogens, such as SARS-CoV-2 or the flu, cause fevers. But as biology professors, we know it’s not that simple. Pathogens cause fevers only indirectly.
When your immune system detects harmful microbes, your body raises its internal temperature to create a hostile environment. Turning up the heat suppresses the proliferation of invaders. In short, the fever is the body’s way of fighting back.
Although many people don’t understand fever’s purpose, animals certainly utilize it. Even so-called “simple creatures,” such as lizards, fish and insects, use fever to recover from illness.
The body’s response
Suppose you catch a virus. The immune system responds by releasing molecules called pyrogens, which induce fever. They signal the brain’s hypothalamus to raise the body’s set point temperature – like adjusting a thermostat.
Normal body temperature hovers around 98.6 degrees Fahrenheit (37 degrees Celsius), but fevers commonly increase temperatures to 100.4-104 F (38-40 C).
When that happens, your muscles contract, causing shivers, and blood vessels constrict to retain heat. You’ll feel cold until your body reaches the new set point, often prompting you to add clothes or snuggle into blankets. When the infection subsides, pyrogens decrease and the hypothalamus resets the temperature. You sweat, your blood vessels dilate, and you cool off. You’re feeling better.
There’s a reason why you shiver when you have a fever.
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Mammals, lizards, fish and insects
Humans are not special in this regard; all mammals are capable of generating fevers. Even without taking their temperature, you might recognize the signs in a familiar companion. When dogs have a fever, they often lose their appetite, become lethargic and may shiver − behaviors that closely resemble how people respond when they’re running a fever.
This adaptive response to infection is widespread in nature. Even cold-blooded animals, which rely on the environment for warmth, raise their temperature behaviorally.
Lizards move to warmer areas when sick. If they’re blocked from doing so − or given fever-reducing drugs − their survival rates drop. Zebrafish swim to warmer waters during infection; a rise of just 5.4 F (3 C) correlates with improved gene expression, stronger antiviral responses and higher survival. Naked mole rats – a social, subterranean cold-blooded mammal that looks like a hot dog with teeth – generate fevers in response to infection, despite their unusual physiology.
Insects, too, show remarkable responses. Desert locusts elevate their body temperature when infected, doing so in a dose-dependent manner: more pathogen, higher temperature. This behavior increases their chance of survival and reproduction.
