The purpose of a vaccine is to stimulate the immune system to recognize and effectively fight infections caused by specific pathogens, such as viruses or bacteria. Vaccines typically contain weakened or inactive parts of the pathogen, such as proteins or sugars. When administered, they train the immune system to recognize and remember the pathogen, enabling faster and stronger responses upon future exposure. This preventative measure significantly reduces the incidence of diseases and can lead to herd immunity within populations.