流感病毒侵入體內，發燒一般都不可避免。為什么一感染這樣的病毒體溫就要升高呢？發燒到底對人體有什么樣的影響呢？

    When your body temperature rises because of an infection, it's called a fever. Fevers are caused by chemicals called pyrogens (熱源) lowing in the bloodstream. Pyrogens make their way to the hypothalamus (視丘下部) in the brain, which is in charge of regulating body temperature. When pyrogens bind to certain receptors in the hypothalamus, body temperature rises.

    One common pyrogen is called Interleukin-1 (IL-1, 白細胞間介素). IL-1 is produced by white blood cells called macrophages（巨噬細胞）when they come into contact with certain bacteria and viruses. IL-1 has multiple purposes, one of which is to signal other white blood cells, called helper T cells, into action.

    One purpose of a fever is thought to be to raise the body's temperature enough to kill off certain bacteria and viruses sensitive to temperature changes. One interesting debate right now, therefore, is, "Should you lower a fever?" Aspirin, for example, will reduce fever; but if the fever is actually helping rid the body of infection, then lowering it might not be a good idea. On the other hand, people sometimes die from fever. Right now the general medical consensus falls on the "reduce the fever" side of the fence.