Pandemic smallpox was responsible for 300–500 million deaths. As recently as the early 1950s, an estimated 50 million cases of smallpox occurred in the world each year. After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the eradication of smallpox in December 1979. To this day, smallpox is the only human infectious disease to have been completely eradicated, and one of two infectious viruses ever to be eradicated. Historically, measles was prevalent throughout the world, as it is highly contagious. According to the U.S. National Immunization Program, 90% of people were infected with measles by age 15.
Human disease modifier gene and shifting from clinical reliance on epidemiological data to patient-specific genetic and biological data. Ultimately, developing therapeutics for human disease informed by an understanding of its molecular mechanisms is challenging. In the case where multi-gene interactions give rise to disease, a series of thorough methodological steps must be taken to determine these mechanisms. For the common case in which a major locus is causally associated with disease, but there exists considerable, unexplained phenotypic variation, this process of discovery typically begins with establishing heritability of the disease phenotype, and the reasonable likelihood that modifier genes contribute to the observed phenotypic variation.