A fever happens when your body temperature goes up higher than normal. It's like your body's alarm system telling you that it's fighting against something harmful - usually germs like bacteria or viruses. Think of it as your body turning up its thermostat to help fight off these unwanted invaders. While most fevers come from common infections like a cold or flu, they can also happen when your body is dealing with other issues, like a reaction to medicine or too much heat exposure. Usually, a fever is your body's natural way of defending itself and will go away once the problem is solved.