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: Live and in sales numbers (2× platinum). The "20th anniversary tour" idea for this album follows on a similar tour in honor of the 20th anniversary of another Megadeth album, "Rust in Peace" (1990) and its associated live album. Fred Thomas of Allmusic commented that while the execution of the album was "surprisingly interesting", the production and feel of the original album were not present. "Countdown to Extinction: Live" sold about 3600 copies in the U.S. during its first week, enough to reach number 119 on the "Billboard" 200. All tracks from "Countdown to Extinction" unless otherwise noted; all music and