When a bacteria or other foreign substance enters the body, phagocytes (specialized white blood cells) ingest the invaders and secrete pyrogens. Pyrogens (fever producing substances) induce the secretion of prostaglandins that reset the hypothalamus at as higher temperature causing vasoconstriction, increased metabolism, and shivering to increase the body temperature. When the stressor is removed, the hypothalamus resets the body temperature to normal. The three phases of fever are: 1. the initial phase, in which the temperature is rising by has not reached the new set point (person feels chill, shivers, and may be generally uncomfortable) 2. The second phase, in which the body temperature reaches its maximum (set point) and remains constant at the new higher level (person feels warm and dry and it may last a few days to a few weeks) 3. The third phase, in which the temperature returns to normal (person feels warm and is flushed as a result of vasodilation, diaphoresis occurs and the fever is �breaking�). The are four types of fever: 1.intermittent, which alternates between regular temperature and fever 2. remittent, which fluctuates wildly but all above normal temperatures during a 24 hour period 3. constant or sustained, which the temperature may fluctuate slightly but is always above normal 4.relapsing, which has short periods of fever with alternating periods of normal temperature each lasting for 1-2 days.