What a Fever Is Actually Doing

The strange thing about being feverish

Being ill with a fever comes with a detail nobody ever explains.

You are burning hot. Somebody puts a hand on your forehead and pulls it back. And yet you are freezing. You want another blanket. You are shivering under two duvets while your skin is hotter than it has ever been.

That combination makes no sense if the heat is simply being done to you. Nobody shivers next to a bonfire.

Then, hours later, it flips. You throw the blankets off, you soak the sheets, and you feel better.

Those two moments — the shivering at the start and the sweating at the end — are the biggest clue there is about what a fever actually is. This episode is about reading them.

"The illness is cooking me"

The natural assumption is that the germ is doing the heating. It got in, it's damaging you, and the temperature is the damage.

If that were true, your body would be fighting the heat. It would be sweating from the very first minute, throwing off blankets, trying to get back down to where it belongs.

It does the exact opposite. At the start of a fever your body makes heat as fast as it can: shivering, which is muscles working purely to generate warmth, and pulling the blood in from your hands and feet so less warmth escapes. Every one of those is a move a cold body makes.

Your body is not resisting the temperature rise. It is causing it.

There is a thermostat in your head

Near the base of your brain sits a small region called the hypothalamus, and one of its jobs is holding your body at a target temperature. It works exactly like the thermostat on a heating system: it has a number it is aiming for, and it turns things on and off to get there.

Too cold? Shiver, narrow the blood vessels near the skin, get the blanket. Too hot? Sweat, open the vessels up, throw the blanket off.

When your immune system spots an infection, it releases chemical signals that travel to that thermostat and turn the target up.

Nothing about the machinery breaks. The thermostat still works perfectly and still does exactly what it always does. It is just now aiming at a different number.

One fever, from start to finish

Two things to watch here, and they are different: the TARGET the thermostat is aiming at, and where your body actually is. What you feel depends entirely on the gap between them. Press to move forward.

Fine — but why bother?

Holding yourself several degrees hot is expensive. It burns through energy at a serious rate, it makes you feel dreadful, and it is hard work. Bodies do not do expensive things for no reason.

The best evidence that the heat is earning its cost comes from an animal that cannot make its own.

A desert iguana has no internal heater. If it wants to be warmer it has to walk somewhere warmer — a sunny rock — and if it wants to be cooler it walks into shade. That makes it a beautifully clean experiment, because a researcher can simply decide whether a warm rock is available.

When infected iguanas were given the choice, they went and sat somewhere hot. They gave themselves a fever by walking. And in a study by Matthew Kluger's group in the 1970s, the infected iguanas that could reach a warm spot survived far better than the infected iguanas kept somewhere cool.

What a fever is, and what it isn't

Six statements. Sort each one, and read what's really going on — a couple of these get mixed up constantly, including by grown-ups.

And 37 was never a magic number

Almost everyone can tell you that normal body temperature is 37 °C — 98.6 °F. It is one of the most confidently repeated facts there is.

It came from a German doctor called Carl Wunderlich, who in 1868 published an enormous study of body temperature and landed on 37 °C. For its time it was a genuinely impressive piece of work.

In 1992 researchers went back and did it again with modern instruments, and got a mean of 36.8 °C — 98.2 °F. Slightly lower. But the more interesting finding was that a single number was the misleading part in the first place: your temperature moves through the day, lower in the early morning and higher in the late afternoon.

That same study put the upper edge of ordinary at about 37.2 °C at six in the morning and about 37.7 °C at four in the afternoon. The same reading can be perfectly ordinary at teatime and a little unusual at dawn.

Doctors generally start calling it a fever from around 38 °C, which is 100.4 °F — but that is a line drawn for practical reasons, not a wall the body knows about.

Now the part that matters most

Everything above is about what a fever is. It is not, at any point, about what to do with one — and that difference is important enough to say outright.

You are not the person who decides whether a fever needs a doctor. That is a grown-up's job, and often a doctor's. If you have a fever, the useful thing you can do is tell somebody, and tell them honestly how you actually feel rather than how you'd like to feel.

Grown-ups are told to get medical help quickly in situations like these: any fever at all in a baby under three months old; a fever that keeps going for days; someone who is very hard to wake, breathing strangely, has a stiff neck, or has a rash that does not fade when you press a glass against it; or anybody who simply seems very unwell — whatever the thermometer happens to say.

That last part is worth repeating, because it is the whole point of the check you just did. How somebody actually is beats the number, every time. Nobody in that list is on it because of a reading.

What to keep