If the body rises in temperature to kill a virus, why can't it lower it also?
The body's Immune System operates most effectively at a higher temperature than normal.
The body has a system, the Immune System, to fight off diseases such as invading viruses and bacteria. But here's the thing - the Immune System is "kept on ice" for the most part, maintaining a low-level of protection. Think of it as the Immune System being on alert but not actually actively fighting anything major.
When a bug does invade, it will typically lie in wait until the numbers are high enough and then it will go from inactive to active. And it's at that point that the Immune System is called into action. To do that, the body raises the temperature to where the Immune System is most active so that it can better fight off the invaders.