no longer appreciate how serious the illness is due to a lack of familiarity with it and become complacent. The lack of personal experience with these diseases reduces the perceived danger and thus reduces the perceived benefit of immunization. Conversely, certain illnesses (e.g., influenza) remain so common that vaccine-hesitant people mistakenly perceive the illness to be non-threatening despite clear evidence that the illness poses a significant threat to human health. Omission and disconfirmation biases also contribute to vaccine hesitancy. Various concerns about immunization have been raised. They have been addressed and the concerns are not supported by evidence. Concerns about immunization safety often follow a pattern. First, some investigators suggest that a medical condition of increasing prevalence or unknown cause is an adverse effect of vaccination. The initial study and subsequent studies by the same group have an inadequate methodology, typically a poorly controlled or uncontrolled case series. A premature announcement is made about the alleged adverse effect, resonating with individuals who have the condition, and underestimating the potential harm of forgoing vaccination to those whom the vaccine could protect. Other groups attempt to replicate the initial study but fail to get the same results. Finally, it takes several years to regain public confidence in the vaccine. Adverse effects ascribed to vaccines typically have an unknown origin, an increasing incidence, some biological plausibility, occurrences close to the time of vaccination, and dreaded outcomes. In almost all cases, the public health effect is limited by cultural boundaries: English speakers worry about one vaccine causing autism, while French speakers worry about another vaccine causing multiple sclerosis, and Nigerians worry that a third vaccine causes infertility. Thiomersal Thiomersal (called "thimerosal" in the US) is an antifungal preservative used in small amounts in some multi-dose vaccines (where the same vial is opened and used for multiple patients) to prevent contamination of the vaccine. Despite thiomersal's efficacy, the use of thiomersal is controversial because it can be metabolized or degraded in the body to ethylmercury (C2H5Hg+) and thiosalicylate. As a result, in 1999, the Centers for Disease Control (CDC) and the American Academy of Pediatrics (AAP) asked vaccine makers to remove thiomersal from vaccines as quickly as possible on the precautionary principle. Thiomersal is now absent from all common US and European vaccines, except for some preparations of influenza vaccine. Trace amounts remain in some vaccines due to production processes, at an approximate maximum of