Thimerosal is the preservative of choice for vaccine manufacturers. First introduced by Eli Lilly and Company in the late 1920s and early 1930s, the company began selling it as a preservative in vaccines in the 1940s. Thimerosal contains 49.6 % mercury by weight and is metabolized or degraded into ethylmercury and thiosalicylate. Mercury, or more precisely, ethylmercury, is the principal agent that kills contaminants. Unfortunately, mercury also kills much more than that.