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 focuses on skydiving. "Captive Honour" is a song about prison and the consequences of crime, while "Psychotron" was written about the Marvel Comics character Deathlok. The album's closing track, "Ashes in Your Mouth", concentrates on the negative aftermaths of war. The longest tune on the record, it is performed at intense tempo and features shredding guitar leads. "Countdown to Extinction" received generally positive reaction from critics. "Spin" wrote that the album "may just be the finest thrash metal album ever made, although purists may find it a bit too polished and easy to swallow". AllMusic reviewer Steve Huey said that