A vaccine is a biological preparation that establishes or improves immunity to a particular disease or virus. The purpose of a vaccine is to prevent a future dangerous infection of a virus by providing your body with immunity to the virus itself. A vaccine is actually a smaller less violent version of the original virus. You are injected with the engineered virus that your immune system is able to destroy. This process generally works because your immune system remembers how to destroy that type of virus and is able to fight off the original virus in the future. Vaccines do not cure you if you are already infected. They are only a preventative method and are usually used to prevent pandemics through what is called “herd immunity” (providing millions of people with vaccines or flu shots). Vaccines do not guarantee complete protection from a disease. Sometimes this is because the host's immune system simply doesn't respond adequately or at all or because the host's immune system does not have a B cell capable of generating antibodies to fight that antigen. Every human body is different and will react accordingly. Some people may have side effects and some may die due to the vaccine itself. Some religious groups do not allow vaccination as it is against their beliefs, and some political groups oppose mandatory vaccination on the grounds of individual liberty.