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.
Countdown to Extinction earlier releases which were "nearly all Mustaine". Mustaine also revealed that producer Max Norman had significant input to the album by making "a lot of suggestions and a lot of great artistic ideas". Guitarist Marty Friedman said that unlike "Rust in Peace", the creation of this record was "completely different". He further stated that the band had changed the songs "a million times" before recording them on demo and entering the studio. The music for "Countdown to Extinction" was written in two different sessions. The first session was done following the conclusion of Clash of the Titans tour, while the