How is a vaccine created?

There are two different ways to make a vaccine: killed and live attenuated.

In making a killed vaccine you grow the actual disease causing organisms, harvest them and treat them with something that kills them and chops them up (chemicals, UV, heat, etc.) These are usually very effective vaccines but often have rare severe side effects.

In making a live attenuated vaccine there are several ways: use a closely related organism that normally does not cause disease in man (this is what Jenner did), use the actual disease causing organism but grow it in a different animal so it causes only a weak disease in man, or use the actual disease causing organism but after harvesting them treat them with something that does not kill them but slows their growth enough that the immune system gets a good chance to finish them off. The biggest drawback here is that sometimes the organisms revert and actually cause the real disease.