The effectivity of vaccines is based on memory cells. Vaccines contain attenuated pathogens or parts of pathogens. They do not harm the person, but instead cause the formation of memory cells.

When the pathogens are passed on to the person, the memory cells recognize them and immediately produce enough antibodies to fight them.

Vaccines can be given to prevent both bacterial and viral diseases. However, their production is challenging due to the constant transformation of viruses and bacteria.