Fever feels unpleasant, so we often turn to medication to lower it. But is that really the best choice? Fever is one of humanity's oldest defenses against illness. What exactly is fever, how does it strengthen the immune system, and should you actually try to suppress it?
The Natural Power of Fever
On Earth, life thrives within certain temperature limits—from -10°C (14°F) in icy pools to 120°C (248°F) in thermal vents. Move beyond these limits, and survival becomes impossible. Every living creature, from microbes to humans, has an ideal temperature range for peak function and a slightly wider, but stressful, range in which they can survive.
As warm-blooded animals, humans work hard to maintain a stable internal temperature of around 37°C (98.6°F). This process, which might seem energy-wasting, serves a defensive purpose—making us nearly immune to one of the deadliest parasites: fungi. Many animals, particularly cold-blooded ones, are susceptible to fungal infections, but human body temperatures are generally too high for these parasites to thrive.
Fever: A Natural Climate Change for Microbes
When microbes like bacteria or viruses invade the body, they find it an environment ripe for infection. Fever acts as a form of "climate change," creating a hostile environment for these invaders. Fever is an ancient defense mechanism that evolved over 600 million years ago and is widespread across animal species. Most animals increase their body temperature when ill. Fish move to warmer waters, lizards bask in the sun, and bees raise the temperature in their hive.
For warm-blooded mammals like us, fever is an internal, rapid response. When pathogens invade, they trigger chemicals called pyrogens, which travel to the brain and raise the body’s thermostat. You may start to shiver as muscles contract to generate heat, while blood vessels near the skin constrict to retain warmth, making the skin feel cool even as your core temperature rises.
The Energy Investment of Fever
Fever requires significant energy, increasing calorie consumption by about 10% per degree Celsius of body temperature rise. It also sends a strong signal to rest, allowing the immune system to focus on fighting the infection.
As bacteria and viruses replicate within your body, fever adds stress, pushing them to their limits. Higher temperatures can damage their cell membranes, disrupt DNA, and slow protein production, making survival difficult. While fever stresses your own cells too, your immune cells, such as neutrophils and killer T cells, become more active and effective at higher temperatures, targeting infected cells and reducing resources like iron and glucose that pathogens need to grow.
Why Fever Outlasts Evolutionary Adaptations
If fever is so effective, why haven’t pathogens evolved to overcome it? Fever creates an evolutionary trap. Pathogens that adapt to survive fever find it harder to infect new, healthy hosts whose bodies are cooler. Fever forces them into a dilemma without an easy evolutionary solution. Some, like the measles virus, adapt by using a “hit and run” strategy, becoming infectious before the immune system fully reacts.
Should You Fight Fever with Medication?
While fever is part of the immune system's defense, we often suppress it with medication. Modern over-the-counter fever reducers like aspirin and ibuprofen are fairly recent innovations, widely available only within the last century. These medications alleviate discomfort, but for many cases, a moderate fever (under 40°C or 104°F) doesn’t pose a threat and doesn’t need to be treated. Exceptions include vulnerable groups, such as pregnant women, the elderly, and those with serious health conditions, for whom fever can be risky.
In some severe infections, studies suggest that fever may improve survival rates, as stopping fever can sometimes worsen outcomes. However, there are important exceptions, particularly in cases involving neurological conditions, where fever could be harmful.
Conclusion: To Medicate or Not?
In general, if your fever isn’t dangerously high and you can tolerate it, allowing it to run its course may actually help your recovery. However, if you’re feeling miserable and are otherwise healthy, taking a fever reducer can provide relief, though it may slightly reduce the immune system's effectiveness.