It's gettin' hot in here: facts about fever

Your heart beats faster. Your fat cells release energy to heat your inner furnace. To reduce the amount of heat that your body loses, the blood vessels under your skin constrict. As your body temperature rises, your muscles begin to contract so fast that your teeth chatter.

Not an illness in its own right

Fever isn't a subtle symptom. Anyone who has had one knows exactly how it feels, and most people experience fever multiple times over the course of their lives. Fever is associated with a whole range of different conditions, from flu to blood poisoning to inflammation of the heart valves. It is often perceived as an aggressive symptom that makes everything worse or – and this is also incorrect – as a separate illness in its own right. Fever is a strategy employed by the body to fight off diseases, and it is gaining an increasing number of advocates in the scientific community.

In a YouTube video, Professor Paul Offit from the University of Pennsylvania says that Hippocrates was right – treating fever is a bad idea. During the pandemic, a group of researchers published an essay entitled “Let fever do its job.” And Märta Sund Levander, professor at Linköping University in Sweden, says: “In many cases fever is not dangerous. It helps.”

Often it is an infection that causes the body’s temperature to rise. If a pathogen or an injury threatens the body, the immune system springs into action. Its defenders include a variety of cells, one type of which are called macrophages. They usually arrive at the scene quickly in order to digest the pathogens directly and activate other defence mechanisms. In the process, they release substances known as pyrogens – which, as the name suggests, are fever-inducers. Pyrogens stimulate the hypothalamus, one of the most important control centres in the brain, to heat up the pathogens. The hypothalamus then ramps up the rate at which energy is burned, and the body’s temperature begins to rise.

The body’s normal temperature is 37 degrees Celsius. At least that is the figure you would hear most often if you asked people on the street. What many don't realise is that this figure is already 170 years old (see box) – and in many cases it is too high. “In fact, the body’s average temperature is closer to 36.4 degrees Celsius,” says Märta Sund Levander. “In up to 25 percent of people, it is even lower.” As part of their research for their book Understanding Fever and Body Temperature, the trained nurse and her co-author took the temperatures of 2,600 healthy people. Her conclusion was that assuming a uniform body temperature for everyone is simply wrong.

Men and women differ

For hormonal reasons, women have higher body temperatures than men, but after the menopause the two converge. Body temperature also changes with age: young people are warmer, and older people cooler. There are also fluctuations over the course of the day, with body temperature being low in the mornings and higher in the evenings (and after exercise or a warm bath, of course, too).

To find your own position on the scale, therefore, you should take your temperature in bed every morning directly after waking up – do this for a while, just like women do to work out their basal body temperature. Always put the thermometer in the same place, says Sund Levander. The armpit or forehead is not a good choice, she warns, as they are too unstable and too strongly affected by external factors. “More reliable locations are your mouth, ear or rectum. I have never understood why so many people prefer the latter.” The highest point of the measurement series is then your benchmark. “If you feel bad and your temperature is one degree or more above that figure, then you could have a fever.”

Fever places considerable demands on the body

Fever is an unpleasant condition. The chills and the accelerated metabolism it causes facilitate heat production and place considerable demands on the body. For every additional degree by which its temperature rises, the body consumes around 10 percent more energy. Fever also puts a strain on the heart and lungs: “If the body temperature increases from 36.5 to 39 degrees, the heart has to work up to 50 percent harder,” says Märta Sund Levander. This is a risk for older people or those who are already in poor health. If the body temperature hits 41 degrees, cells start to die. In other words, fever can also be fatal. 

Why does the body take such a high risk? Over the course of our evolution, fever appears to have been a double-edged but reliable sword. Its origins can be traced back up to 600 million years.

We know that fish which have contracted infections swim into warmer waters. If sick desert iguanas are unable to sit on hot stones, their chances of survival decrease by 75 percent. There are a wide variety of reasons for this. Some viruses do not tolerate high temperatures. Heat prevents bacteria from multiplying so fast. However, the main reason why the body adjusts its thermostat in this way is that a warmed-up immune system functions more effectively. At temperatures of between 38 and 40 degrees, the immune cells can communicate more easily and multiply faster.

Despite this, there seems to be a consensus within society that a high temperature should be brought down if at all possible. This may be because the drugs that do this can be found in almost every medicine cabinet and taking them gives us the feeling that we are doing something to fight the illness – which isn't always true. Furthermore, not having a high temperature simply feels better. This isn't the right approach to take, warns Paul Offit from the University of Pennsylvania. Fever sufferers should keep warm and sit out the infection, rather than running around outside and infecting others.

However, caution is advised if the patient develops unusual symptoms or their behaviour changes noticeably, says Märta Sund Levander. A dry nappy in small children is also a warning sign, she adds. To put parents’ minds at rest: while their immune system is still developing, children will often get fevers, but out of a hundred children with a high temperature only one is suffering from a serious illness. In many cases, therefore, it is sufficient for them simply to stay in bed and have plenty to drink. The immune system then has free rein to do its job – as it has been doing for millions of years, in fish, desert iguanas and even human beings.