A virus is intertwined within our body's own cells. The vaccine works with something called molecular mimicry, which means the body sees those viruses thinking that it's foreign, but actually finds pieces of those viruses that match pieces of our healthy cells. Put another way... the vaccine causes the immune system to be unable to tell the difference between the body's healthy cells and the cells that have been affected by the virus. Therefore, our immune system will turn on itself leading to an autoimmune condition because it cannot differentiate between what it's being told to reject and itself.