Vaccines are substances that artificially induce or maintain an immune response against an infectious agent. Since Edward Jenner first developed the smallpox vaccine in 1796, humans have effectively responded to a large variety of infectious diseases caused by viruses, bacteria, and parasites. Vaccines have reduced and in some cases even eliminated serious human diseases

To date, vaccines are available for over 30 infectious diseases. Depending on the particular characteristics of a disease, vaccines can be divided into three types: inactivated vaccines, which use inactivated external pathogens or toxins as an antigen; live attenuated vaccines, which use attenuated pathogens as the antigen; and subunit vaccines, which use purified antigens.

Despite the fact that many vaccines have been developed, there are still many diseases for which no vaccines are available. These include malaria, dengue fever, and the human immunodeficiency virus (HIV) infection. Much effort is being made to develop vaccines for these diseases. Research is focused on developing vaccines against cancer and chronic diseases such as diabetes, hyperlipidemia, and Alzheimer's. There is even interest in developing vaccines that negate the effects of narcotics to treat addiction.

In addition, new technologies aimed at maximizing the effects of vaccine antigens, such as new adjuvants and antigen delivery tools, and combining them with existing vaccines, are also being developed.