There are a number of infectious diseases -- typhoid fever and Rocky Mountain spotted fever, for instance -- that have "fever" in their names. That said, the word fever actually refers to a symptom, rather than a specific illness.
When you get a fever, your body temperature, typically around 98.6 degrees Fahrenheit (37 degrees Celsius), rises by at least 1 degree. A temperature of 100 to 102 degrees F (37.8 to 38.9 degrees C) is considered a low-grade fever. If it rises above 103 degrees F (39.4 degrees C), it's considered a high-grade fever, which is a sign that something really bad may be happening.
A variety of conditions, from cancer and multiple sclerosis to inflammatory bowel disease and heat exhaustion, can cause your temperature to soar, which is why developing a fever is a good reason to see a doctor. But odds are that your body is fighting off some nasty viral or bacterial invader.
The attacking microbes are spotted by the immune system, which sounds the alarm by releasing chemicals called cytokines into the bloodstream. When the cytokines reach the hypothalamus, an almond-sized structure in the brain that acts the body's internal thermostat, they inhibit heat-sensing neurons and excite cold-sensing ones. The hypothalamus is fooled into thinking that the body's temperature is too low. It cranks up the heat by telling the skeletal muscles to generate warmth by contracting rhythmically -- to us, that's shivering -- and constricting the blood vessels in the skin, so that the heat is retained.
While you feel awful, a fever is a potent anti-infection weapon. The heat stimulates white blood cells to produce antibodies to fight the invaders. And some bacteria and viruses can only tolerate a narrow temperature range, so heat can kill them or inhibit their reproduction.
In the next section, we'll look at where the idea of starving a fever came from.