Many vaccines are offered in multi-dose vials. When doses are drawn from such vials, they are contaminated with live and dead bacteria, viable and inactive viruses, viable and dead fungus and aeroallergens. They also were contaminated with latex from the latex vial stoppers. Thimerosal is used as a preservative in such vaccines to kill bacteria, fungus, and inactivate viruses. However, infection due to viable bacteria, fungi and viruses are not the only danger in contaminated vaccines. Immunization to proteins from dead pathogenic bacteria and viruses in vaccines may offer either a protective effect or cause diseases such as allergies or autoimmunity. Immunization against commensal bacteria, fungi, latex or aeroallergens are definitely harmful. They cause the development of inflammatory bowel diseases, gastroesophageal reflux disease, atopic dermatitis, seasonal allergies, latex allergy and allergic asthma. Similarly, many vaccines have to be reconstituted using diluents. A process that also introduces contaminants as in the multi-dose vial case. Vaccine recipients will develop immune responses against ALL these antigens. Immune responses against pathogens are unlikely to be protective because the immune response differs from natural exposure via a natural route of exposure. Such an altered immune response may even be harmful. Injecting proteins causing the development of allergies has been known for a hundred years following the Nobel Prize winning discovery by Dr. Charles Richet. The fact that thimerosal is used in vaccines is an acknowledgment of the occurrence of contamination. However, the fact that this mechanism of disease causation by non-target protein contamination of vaccines is not even discussed in the vaccine literature and the fact that vaccine makers and regulators have ignored this for decades, is proof of their incompetence. Multi-dose vaccines are more likely to be used in mass vaccination campaigns. Vaccination of health care workers are likely to use multi-dose vaccines. This can explain why health care workers have the highest rates of asthma.