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 "Megadeth guns for arena thrash success and gets it on "Countdown to Extinction"". However, Huey noted the simplification of Megadeth's sound on the album in response to Metallica's commercially successful self-titled album released the previous year. Sputnikmusic reviewer Mike Stagno praised Mustaine's vocals as well as a number of the album's tracks. Despite calling this effort "Megadeth's first step into the world of mainstream music", he also noted that "the band still maintains its identity". Reflex described the sound as "impressively clean, streamlined, and sharp, brimming with killing riffs". They highlighted the vocal delivery on the record, saying "Mustaine's never