An inactivated virus is a virus that has been killed or inactivated so that it can no longer replicate or cause disease. This process is often achieved through heat, chemicals, or radiation. Inactivated viruses are commonly used in vaccines, allowing the immune system to recognize and respond to the virus without the risk of infection. Vaccines made from inactivated viruses, such as those for polio and hepatitis A, help the body develop immunity. When a person receives an inactivated virus vaccine, their immune system learns to identify the virus, preparing it to fight off future infections without exposing the individual to the actual disease.