Question: Why does injecting a small amount of a disease into a person help prevent it?

Helen Fletcher answered on 21 Jun 2011:

Hi, it does protect you! but you have to disable the disease in some way so it doesn’t establish an infection. This can be done by growing it for many years in a petri dish (this is how very early vaccines were made). When you grow the disease in the lab in a petri dish it starts to lose the genes it needs for infecting a human. After many rounds of growth you end up with a virus/bacteria which is similar enough to the disease causing organism to be able to induce a protective immune response but different enough from the disease causing organism so it can’t make you sick. Nowadays we can speed up this process by genetically modifying bacteria and viruses to delete genes out. You can also inject a heat killed pathogen, and that will also protect you. If you are not bored read on…..! It protects you because you are making a “memory” immune response. When your immune system has seen a disease it makes B and T cells which circulate in our blood for the rest of our lives and can remember the disease. When you get exposed to the disease a second time these expand very rapidly and clear the infection before you get sick. A vaccine basically tricks the immune system into thinking it’s seeing a disease and causes the immune system to make a “memory” response to that disease. Very clever!!