Briefly describe how a vaccine works.

A vaccine is a dead or attenuated (weakened) form the pathogen. When injected into the host, the antigens on the surface of the pathogen generate an immune response. This stimulates the production of white blood cells which divide into various types of cells (e.g B-helper cells, T-killer cells, and B-memory cells). These memory cells linger in the blood stream and mean that if the host is ever infected with the same pathogen again, the memory cells divide and produce antibodies to fight the disease before the host displays symptoms.