What happens inside your body when you get a fever?

When you get a fever, your immune system is actively fighting an infection. White blood cells detect pathogens and release signaling proteins called pyrogens, including cytokines like interleukin-1. These travel to the hypothalamus in your brain, which acts as your body's thermostat. The hypothalamus responds by raising your body's temperature set point, triggering mechanisms like shivering to generate heat and blood vessel constriction to retain it. The elevated temperature serves a purpose: it slows the reproduction of many bacteria and viruses while speeding up immune cell activity. Your metabolism increases, which is why you feel exhausted. Once the infection is cleared, pyrogen levels drop and the hypothalamus resets to normal, causing sweating to release the built-up heat.