An inactivated virus is a virus that has been killed or rendered non-infectious through various methods, such as heat or chemicals. This process ensures that the virus cannot replicate or cause disease, making it safe for use in vaccines. Inactivated viruses stimulate the immune system to recognize and respond to the virus without causing illness. Vaccines made from inactivated viruses, like the ones for polio and hepatitis A, help the body develop immunity. When a person is vaccinated, their immune system learns to identify the virus, preparing it to fight off future infections.