this answer is provided by Jonathan Blum
Division of Infectious Diseases,
Kaiser Permanente Medical Center
(former HHMI physician postdoctoral fellow)
The hypothalamus, a part of the brain just above the pituitary gland, contains the body's "thermostat." It senses the temperature of the blood and compares it with a "setpoint"—the desired temperature. If the actual temperature differs from the setpoint, it tries to adjust the temperature. This is similar to what a thermostat in your house or car does. However, it can't just turn on the furnace or the air conditioner. Instead, it sends out signals that cause us to sweat if we are too hot or to shiver if we are too cold. (Sweating cools the body as the sweat evaporates; shivering warms the body, because much of the energy used by shivering muscles is converted to heat.) In addition, it causes us to feel uncomfortable, which prompts us to seek out a source of heat or cold. For example, if your body temperature is too low, you'll look for a warmer place or some blankets. If it's too high, you'll look for a cooler spot or reduce your exercise level (since exercise generates heat).
When you develop a fever, the body sends signals to the hypothalamus that tells it to raise the setpoint to a higher temperature. Often, but not always, this is a response to an infection, such as a bacterium or a virus. The immune cells that respond to the infection secrete molecules called "endogenous pyrogens." (Endogenous means made by the body, and a pyrogen is something that causes the temperature to rise.) The endogenous pyrogens travel through the blood and reach the hypothalamus, where they cause the setpoint to increase. Once that happens, the hypothalamus tells the body, "You're too cold." Remember that the temperature may be normal in this situation, but what matters is that the temperature is lower than the setpoint, which has been raised. So you feel cold, get under the blankets, and shiver.
A while later, when the temperature has risen to the setpoint, the hypothalamus sends out the signal, "Your temperature is fine now." Then you no longer feel cold and often throw off some of the blankets. And when the fever "breaks," the level of endogenous pyrogens decreases, the hypothalamus changes its setpoint back to normal, and now the elevated temperature again seems too hot. Suddenly you start to sweat and throw off all the blankets! Soon this leads to a decrease in temperature back to the normal setpoint, and you feel normal again.
So you actually feel coldest when the temperature is starting to rise and hottest when it's starting to fall. This seems strange, but it's all the hypothalamus's way of getting you to alter your behavior to change the temperature to a new setpoint. When your temperature is at the setpoint—whatever it is—you tend to feel normal, even if the actual temperature is high. What you perceive is not the temperature itself, but the difference between the setpoint and the actual temperature.
Elevated temperature resulting from exercise (such as heat exhaustion) is different. In this situation, the body temperature rises because of hot weather or heavy exercise. Because the body is not producing pyrogens, the hypothalamus does not raise its setpoint. So people whose temperature rises during exercise typically feel hot, not cold, because their temperature is above the setpoint. (In heatstroke, they may not feel hot, but that's because the temperature is so high that the hypothalamus actually malfunctions.)
When my patients come to me and say they've had a fever, the first thing I ask them is, "What do you mean?" Often people think they should feel warm when they have a fever, but as you've pointed out, as the temperature goes up, they feel cold, and when it stays high, they may feel normal (until it falls). So I always ask about their symptoms and whether they've actually measured their temperature. In my experience, most patients are not very good at knowing when they've had a fever. I also ask if they've had shakes or chills or if they've had sweats—all signs of changing temperatures. Of course, people may shake from fear or sweat from exercise, so the answers have to be interpreted in terms of the whole situation.
Why do we have fevers? The most likely answer is that fever represents part of the body's immune response to infections and that it is somehow involved in controlling the infection. The details of this are not fully understood. The point is that the fever is generally not harmful in itself, unless it's very high.
