Vaccines contain weakened or killed pathogens that stimulate the body's immune response by producing antibodies and memory cells specific to the disease. This provides immunity, protecting individuals from illness if exposed to the pathogen again. Vaccinating a significant portion of the population also provides indirect protection to those who cannot develop immunity through herd immunity. The first vaccine was developed by Edward Jenner who observed cowpox provided protection from smallpox, hypothesized this could be tested by inoculating people with cowpox, tested his hypothesis, and concluded vaccination provided acquired immunity.