Certain vaccines are medical preparations designed to provide immunity against specific diseases. They work by introducing a harmless part of the virus or bacteria, known as an antigen, into the body. This prompts the immune system to recognize and remember the pathogen, enabling it to respond more effectively if exposed in the future. Common examples include the influenza vaccine and the measles, mumps, and rubella (MMR) vaccine. Vaccines can be classified into two main types: inactivated (killed) vaccines and live attenuated (weakened) vaccines. Inactivated vaccines, like the polio vaccine, contain pathogens that have been killed, while live attenuated vaccines, such as the yellow fever vaccine, use weakened forms of the virus. Both types help the body build immunity without causing the disease.