Fever Feels Horrible, but is Actually Awesome!

Fever is one of the oldest and most effective defenses our bodies have against disease. While it may feel unpleasant, fever is actually a critical part of our immune system’s strategy for fighting off infections.

In this post, we’ll dive deep into the science behind fever – what it is, how it works, and why it’s so important for our health. We’ll also explore the debate around whether we should be trying to suppress fever with medication, or letting it run its course.

The Heat of Life

Life on Earth is able to thrive in a relatively narrow temperature range, typically between -10°C and 120°C. Step outside of this range and most organisms will quickly perish.

Every living thing, from microbes to mammals, has an ideal temperature range where their cells and internal systems function best. For humans, that ideal temperature is around 37°C or 98.6°F.

Maintaining a stable core body temperature of 37°C requires a significant energy investment from our bodies. This may seem wasteful, but it’s actually a defensive adaptation that makes us almost entirely immune to one of the worst killers and parasites – fungi.

Most cold-blooded animals and their internal environments are easily infected by fungi. But our toasty 37°C bodies are simply too hot for most fungal pathogens to survive in.

This brings us to the topic of fever. Fever is the body’s way of ramping up the heat even further as a defensive measure against invading microbes. It’s a form of “defensive climate change” that pushes pathogens outside of their ideal temperature range, making the environment inside our bodies inhospitable for them.

Fever has evolved in some form in most animals, from fish swimming into warmer waters, to lizards basking in the sun, to bees heating up the air inside their hives. As warm-blooded mammals, we have even more drastic options at our disposal.

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When Your Blood Turns Into Lava

When our bodies are invaded by bacteria, viruses, or other pathogens, fever is one of our first lines of defense. It’s triggered by a diverse group of chemicals called “pyrogens” – “the creators of heat” – that float from the site of infection to the brain.

These pyrogens are detected by specialized receptors in the brain, which then crank up the body’s internal thermostat. First, we start to shiver as our skeletal muscles contract rapidly to generate heat. At the same time, our blood vessels near the surface of the skin constrict to prevent heat from escaping.

The result is a rapid rise in our core body temperature, while the skin feels cool to the touch. Fever is a systemic, whole-body response that requires a significant energy investment – we burn about 10% more calories for every degree Celsius our temperature rises.

But this energy expenditure is a small price to pay compared to the benefits fever provides in the fight against infection. When bacteria and viruses invade our bodies, they are typically trying to be stealthy and multiply as quickly as possible. Fever throws a massive wrench in their plans.

The intense heat generated by fever can cause all sorts of problems for pathogens:

It can damage their cell membranes and organelles, causing their internal machinery to break down.
It can disrupt their DNA and protein production, hampering their ability to replicate.
It creates a highly stressful environment that forces them to divert precious resources away from growth and reproduction.

Essentially, fever turns the human body into an inhospitable, hostile environment for our microbial invaders. And the best part is, our own cells are much better equipped to handle the heat than the pathogens are.

Fever Boosts the Immune System

While fever does put stress on our own cells and organs, there is one system that actually thrives in the heat – our immune system.

Fever accelerates the recruitment and activity of key immune cells like neutrophils, macrophages, dendritic cells, and killer T cells. These cells become more effective at detecting, engulfing, and destroying pathogens.

Fever also makes it harder for the invading microbes to access the resources they need, like iron, glucose, and glutamine. It turns the “battlefield” inside our bodies into a nutritional desert, starving the pathogens.

Viruses, in particular, are extremely sensitive to heat. The common cold virus, for example, can only infect the relatively cooler respiratory tract – it can’t survive the higher temperatures found elsewhere in the body, especially during a fever.

Infected cells also suffer greatly during a fever. As their internal machinery starts to break down from the heat stress, they panic and produce massive amounts of heat shock proteins in a desperate attempt to repair the damage. But this ends up being a trap.

You see, heat shock proteins are a signal to our natural killer cells and killer T cells that something is wrong. They use these proteins to identify and destroy infected cells, along with all the viruses contained within. The pathogens’ own survival mechanisms end up being their downfall.

Outsmarting Evolution

If fever is such an effective weapon against pathogens, you might wonder why they haven’t simply evolved to become resistant to it. After all, it’s been around for at least 600 million years.

The answer lies in the evolutionary dilemma fever creates for microbes. If a pathogen manages to survive a fever long enough, the individuals that are better adapted to deal with the heat will reproduce more. Over time, this creates a population of heat-resistant microbes.

However, this adaptation then becomes a handicap when those pathogens try to infect new hosts. Healthy human bodies are simply too cold for the heat-loving microbes to thrive in. They now have to compete with their own “cooler” cousins that are better suited for infecting normal body temperatures.

This evolutionary catch-22 means pathogens can’t simply evolve their way out of the fever problem. Some, like the measles virus, have resorted to “hit-and-run” tactics instead. They replicate at an ultra-fast rate and become most infectious right before the full force of the fever hits, allowing them to do their damage before being beaten back.

Fever, then, is a crucial part of the puzzle that is our immune system’s multi-pronged defense against invaders. It attacks pathogens from multiple angles, making it incredibly difficult for them to adapt.

Should You Fight Fever with Medications?

Given how effective fever is at combating infections, you might be wondering – why do we so often try to suppress it with medications like aspirin or ibuprofen?

The truth is, the ready availability of over-the-counter pain relievers is a relatively new phenomenon in human history. For most of our past, we simply had to endure the discomfort of fever and other symptoms when we were sick.

Pain and fever are closely linked, and medications that reduce pain also tend to lower body temperature. This has led to a common practice, especially with children, of trying to aggressively treat any fever that arises.

However, the evidence suggests that for fevers below 40°C (104°F), the benefits of letting the fever run its course likely outweigh the drawbacks. Fever is not a disease in itself, but rather a sign that the immune system is actively fighting an infection. Suppressing it may actually slow down the healing process.

There are certain exceptions, of course. Pregnant women, seniors, and severely immunocompromised patients may be at higher risk of complications from a fever and should consult their doctor. And any fever over 40°C is a sign that the body’s temperature regulation system is malfunctioning and requires medical attention.

But for otherwise healthy individuals, the decision of whether to treat a fever comes down to personal preference and tolerance. If you can bear the discomfort, letting the fever run its course may help your body recover faster. But if the fever is making you miserable, taking medication to reduce it is a reasonable choice as well.

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Conclusion

Fever may feel unpleasant, but it is actually one of the oldest and most effective weapons in our immune system’s arsenal. By raising our body temperature, fever creates a hostile environment that pathogens struggle to survive in.

At the same time, fever supercharges the activity of our immune cells, helping them detect, attack, and destroy invading microbes more effectively. It’s a multi-pronged defense strategy that has evolved over hundreds of millions of years.

While there are some cases where fever should be treated, for most healthy individuals, letting it run its course may be the best approach. Fever is a sign that your body is working hard to fight off an infection – and it’s winning that battle in large part thanks to the heat.

So the next time you’re burning up with a fever, take comfort in the knowledge that your enemies are having a much worse time than you. Your body’s defenses are hard at work, and fever is one of their most powerful weapons.