In a memorable 2015 TED talk, Bill Gates told his audience that “when I was a kid, the disaster we worried about most was a nuclear war.” But today, he said, “if anything kills over 10 million people in the next few decades, it’s most likely to be a highly infectious virus, rather than a war. Not missiles, but microbes.” (p.30)
These diseases have a constant presence in a region. Colds and seasonal flus are endemic because they are expected, as is malaria in some tropical regions.
These often sudden outbreaks occur when the tally of new cases exceeds what is expected for an infectious disease in a given region.
Like the H1N1 flu of 2009, pandemics not only exceed expected case levels but also spread over many countries―and often many continents too. (p.27)