Despite the large degree of success of many vaccines, humans are still plagued by diseases such as AIDS, chronic hepatitis, and cancer. Together these three diseases are responsible for over 530,000 deaths per year in the United States, 94% of this number being attributed to cancer alone. Without an effective vaccine, education is one of the few ways available for controlling disease. For example, education about high-risk behavior has helped bring the 78,000 reported cases of AIDS in the United States in 1993 down to 43,000 reported cases in 1999. Alterations in diet and lifestyle and having regular checkups can aid in lowering the reported cases of certain cancers. Nevertheless, the development of an effective vaccine would be the optimal solution to lowering incidence of AIDS and cancer.

Increasing knowledge of the human immune system, together with understanding of the mechanism by which vaccines provide immunity and how pathogens infect and evade the host defenses, allows for a focused approach on the development of effective vaccines. In the search for a vaccine, the cost of manufacture, ease of delivery and transport, and effectiveness at directing the immune response toward the pathogen with little negative impact on the host are taken into account. The effectiveness of a vaccine depends on the type of vaccine as well as the characteristics of the target pathogen. A large variety of vaccine strategies are available, and new delivery systems are being developed. Since different pathogens exhibit a variety of mechanisms for evading the human immune response, defining the immune response that would be most effective against a particular pathogen is also an important step in vaccine development.