Note: I think this question's solutions were posted twice

Q: When we have a fever, our body temperature has increased it in response to chemicals known as pyrogens. Pyrogens are produced by bacteria that infect the body and by the immune cells that combat the infection. The pyrogens act on the hypothalamus, which is the controller for body temperature, and increase the body temperature set point. Explain the process by which fever develops, and the symptoms that we associate with fever.

A: The pyrogens reach the hypothalamus where they change the setpoint (fever is also known as pyrexia). Although the body temperature is that the value that is considered normal, it is below the current set point, and so the negative feedback system begins increasing body temperature to the new set point. Even though your body temperature is apparently normal, you will still feel cold and you will shiver in order to generate heat.

Q: When we have a fever, our body temperature has increased it in response to chemicals known as pyrogens. Pyrogens are produced by bacteria that infect the body and by the immune cells that combat the infection. The pyrogens act on the hypothalamus, which is the controller for body temperature, and increase the body temperature set point. Explain the process by which fever develops, and the symptoms that we associate with fever.

A: In fever there is an increase in the set point for body temperature. Although the person's body temperature is initially at the level considered normal, it is below the set point, so the negative feedback system operates to increase body temperature to the new set point. When the patient's body temperature is below the set point a person will feel cold, even if their temperature is higher than normal. Shivering helps to increase body temperature because the conversion of energy released during ATP breakdown (the energy that powers muscle contraction) is quite inefficient. Only some of the energy goes to muscle contraction and the rest generates heat.

Pyrogens are not the only things that change the set point. There is a diurnal variation - the body temperature of a healthy person increases by about 0.5° - 1.0°C from morning to late afternoon. Body temperature also tends to be higher in women than in men and is approximately 0.4°C higher during the second half of the menstrual cycle compared with the first. Younger people also tend to have higher body temperatures than older people.