Claims that childhood vaccines cause autism stem from the presence of mercury, a heavy metal that was used as a preservative to keep viruses and bacteria alive in vaccines. Many families of autistic children typically suspect this around 18 months of age, which coincides with the timing of the combined MMR vaccine in the routine vaccination schedule. However, a report published by the World Health Organization clarifies that ethylmercury (thiomersal) has a half-life of less than a week in the body, and that mercury-related chronic illnesses are not observed.