Vaccines are considered to be one of the most effective strategies for the prevention of many infectious diseases. Attenuated or live microorganisms comprising both pathogenic and commensal bacteria are engineered to deliver heterologous antigens to elicit host immunity. Vaccines are capable of inducing a broad range of immune responses including localized mucosal, as well as systemic humoral or cell-mediated immunity. Attenuated vaccine technology has been used to elicit immune responses against bacterial, viral, protozoan and metazoan pathogens. The other common types of vaccines that are prescribed by practitioners are inactivated vaccines, subunit vaccines, toxoid vaccines, conjugate vaccines and DNA vaccines. Recombinant vaccine vector is a significant breakthrough in the vaccine discovery.