Clearly you don’t… Ok ahem. Vaccines come in several types, the measles vaccine (MMR) is a live vaccine meaning it uses a living but weakened version of the virus to train the immune system. This is done through the immune system cells recognizing the antigens of the virus it just killed. Now whenever a similar virus comes along, the immune system is already prepared and sends out B cells and T cells to kill that new virus, thus preventing infection. It doesn’t remain fucking dormant.