Final answer:
Vaccines have played a crucial role in preventing diseases like poliomyelitis and chicken pox, leading to significant reductions in their prevalence worldwide.
Explanation:
Poliomyelitis and chicken pox are diseases that can be prevented by vaccines. Vaccination has been instrumental in reducing the incidence of various diseases worldwide, such as smallpox and polio, which were once widespread but have been significantly controlled through immunization programs.
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