There are multiple types of vaccines: vaccines containing dead microorganisms (e.g. the influenza and cholera vaccines); vaccines containing attenuated microorganisms (e.g. the MMR vaccine); vaccines containing toxoids (e.g. the DPT vaccine); vaccines containing subunits of the pathogen (e.g. the HPV vaccine); and nucleic acid (e.g. RNA and DNA) vaccines (e.g. avian flu vaccine, West Nile Virus vaccine, and multiple cancer vaccines). Vaccine adjuvants are agents that stimulate the immune system and increase the response of the immune system to a vaccine.