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 genetic resistance to malaria fetal hemoglobin (HbF) confer some protection against falciparum malaria in adults with Hereditary persistence of fetal hemoglobin. Evolutionary biologist J.B.S. Haldane was the first to give a hypothesis on the relationship between malaria and the genetic disease. He first delivered his hypothesis at the Eighth International Congress of Genetics held in 1948 at Stockholm on a topic "The Rate of Mutation of Human Genes". He formalised in a technical paper published in 1949 in which he made a prophetic statement: "The corpuscles of the anaemic heterozygotes are smaller than normal, and more resistant to hypotonic solutions. It is at least