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 involve large human populations in which genetic modifiers cannot be experimentally manipulated. Thus, in human populations the roles of genetic modifiers are often established by statistical implications, without evidence for causation or opportunity for functional characterization. The study of human disease modifiers and oligogenic disease is a hybrid field that can yield unique and comprehensive insight into complex mechanistic causes of human disease, necessary for the development of effective therapeutics Characterizing complex multi-gene interactions responsible for disease- and further, developing effective therapeutics- is exceedingly difficult, and the molecular mechanisms of few oligogenic diseases have been elucidated. Retinitis pigmentosa is one