One of the best-known vaccine controversies involves thimerosal. Thimerosal is a preservative that contains ethylmercury. It has been used since the 1930s as a way to prevent bacteria from growing in a vial of vaccine. Before thimerosal was used, medical providers might inadvertently introduce bacteria into a multidose vial, thus contaminating the remaining vaccine. When a later dose of vaccine was drawn up from the vial, it would contain bacteria that could infect, or possibly even kill, the next vaccine recipient.