Most flu shots do not contain a live virus. Instead, they use inactivated (killed) virus particles or just a single protein from the virus. When you get the shot, your immune system detects these harmless pieces as foreign invaders. It then gets to work creating special proteins called antibodies specifically designed to fight that flu strain. This process is like a training drill for your body's defenses. If you're later exposed to the real, active flu virus, your immune system already recognizes it and can launch a fast and strong attack, preventing you from getting sick.