How is vaccine effective as a preventive measure?

Explanation

A vaccine consists of killed or attenuated microbes or their products. It retains antigenic property but not pathogenicity. Thus, it induces specific immune response in an organism. Memory cells are also produced as a result of immune response. These memory cells persist in the body for many years. If the disease-causing pathogen attacks, then the person.s immune system recognises and destroys it, before the pathogen can produce illness.