You feel a cold coming on. By evening, the thermometer says your temperature is up, yet you're curled under a pile of blankets, shivering like it's January. How can your body be running hot and feel freezing at the same time?
The short answer is that a fever starts with your brain raising its target temperature. Your body is suddenly cooler than that new target, so it feels cold and shivers to generate heat. The chills are your body working to warm itself up to the new setting.
The Thermostat in Your Head
Body temperature is controlled by a small region of the brain called the hypothalamus. It works a lot like the thermostat on your living room wall. Normally it holds your temperature steady around a set point. That normal temperature isn't one fixed number for everyone, either. It shifts with age, time of day, activity, and other factors.
When you catch an infection, your immune system releases chemical messengers. These lead to the release of a substance called prostaglandin E2, which acts on the hypothalamus and nudges the set point higher. Anything that triggers a fever this way is called a pyrogen.
Why You Feel Cold
Picture turning your home thermostat from 68 to 75 degrees. For a while, the house feels chilly compared with the new setting, and the furnace kicks on. Your body does the same thing.
First, blood vessels near the skin tighten, a process called vasoconstriction. That keeps heat from escaping, but it also makes you feel cold. Then your muscles start contracting rapidly. That's shivering, and it produces heat by raising your metabolic rate.
Your body may also release hormones such as epinephrine to boost heat production. When chills come with violent shivering, doctors call them rigors. Chills that happen without a high fever are normally mild.
Babies have an extra trick. In infants, the body can switch on a special type of fat called brown adipose tissue, which produces heat without any shivering at all.
And Then Comes the Sweat
Eventually, the set point drops back to normal, either on its own or with medicine. Now the opposite problem appears. Your body is warmer than its target, so it switches to cooling mode.
Blood vessels near the skin widen, shivering stops, and you start to sweat. That's the moment many people throw off the covers and say their fever just broke.
Is a Fever Actually Helpful?
A fever caused by an infection is generally considered protective. Studies in warm-blooded animals suggest they recover from infections more quickly with a fever. Fever has been recognized as a sign of illness, rather than a disease itself, since around the time of Hippocrates more than 2,000 years ago.
Other creatures use heat too. Some grasshoppers move to warmer spots to raise their body temperature a few degrees and slow the growth of harmful fungi. Honeybee colonies can even raise the temperature of the hive in response to a fungal parasite.
Most people with a fever recover without special treatment, but some situations need medical care, including babies younger than three months. If you're worried about a fever, talk to your doctor.
So the next time you're shivering under a quilt with a fever, know that your body isn't confused. It's following new orders from headquarters, turning up the heat to help you fight off whatever bug you caught. A warm cup of tea can't hurt either.
