There r 2 types of vaccines. Conventional vaccines contain either killed or weakened from of the very organisms which cause the disease. These vaccines have less effect. Another type of vaccines is synthetic vaccines protect mankind from many diseases. Some of the vaccines protect human beings from deadly diseases. The vaccines activate the body's defence machinery by alerting the brigade of the white cells which play important role in killing any invading foreign organism. The white cells produce antibodies which work like missiles anthus they protect the mankind. Scientists refine the vaccines with the help of the understanding of the response of the body to the vaccine and the understaning of the disease causing organism.