An adjuvant is an agent that lacks a specific antigenic response that, however, does stimulate an immune response to another agent, typically a vaccine. Adjuvants are commonly used with vaccines to enhance the immune response to the vaccine, a strategy used to reduce the rate of vaccine failure. Vaccines by their nature are not the natural disease for which immunity is desired, and thus can have less than ideal rates of immune response unless carefully designed. The value of adjuvants in boosting immune responses was discovered by serendipity when it was found that contamination of experimental vaccines enhanced, rather than diminished, their capacity to produce an immune response. Currently, the most common adjuvants used for human vaccines are inorganic aluminium salts, aluminium hydroxide and aluminium phosphate.