We humans are grateful to the Almighty for granting us the noblest kind of life on earth. At the same time, he has given the cause for worry by making us susceptible to various infection and diseases. To defend ourselves against diseases, he has given us two armaments too. One is cleverness. The other one is a certain capacity of the body to oppose diseases. The capacity may be different from individual to individual, and from time to time. With age the natural power of confrontation to diseases may decrease, or may even be lost. The confrontation is termed 'immunity'. Immunity is a medical condition conferred on the body by the attendance, or formation, of certain substances, called antibodies, which are composed of proteins.
With growth in medical science, it has become possible to create immunity to specific diseases by introducing certain substances, called vaccine, into the body. Vaccines have been developed against a huge number of diseases. With extensive use of vaccines, it has been possible to increase the expectation by a large percentage in all parts of the world. The death rate of infants has been lowered vividly with no side effect. It has been possible to trim down the incidence of many fatal diseases, and at least one disease, small pox, has been eradicated totally from all parts of the world.