What are the catastrophic risks, and how catastrophic are they?
The number of extreme catastrophes that have a more than negligible probability of occurring in this century is alarmingly great, and their variety startling. I want to describe them and in doing so make clear the importance of understanding what science is doing and can do and where it is leading us. I begin with the natural catastrophes and move from there to the man-made ones, which I divide into three groups: scientific accidents, other unintended man-made catastrophes, and intentional catastrophes. The 1918–1919 flu pandemic is a reminder that nature may yet do us in. The disease agent was an unexpectedly lethal variant of the commonplace flu virus. Despite its lethality, it spread far and wide because most of its victims did not immediately fall seriously ill and die, so they were not isolated from the healthy population but instead circulated among the healthy, spreading the disease. No one knows why the 1918–1919 pandemic was so lethal, although it may have been due to a combination of certain features of the virus’s structure with the crowding of troops in the trenches and hospitals on the Western Front (where the pandemic appears to have originated near the end of World War I), facilitating the spread of the disease. The possibility cannot be excluded that an even more lethal flu virus than that of the 1918–1919 pandemic will appear someday and kill many more people. There is still no cure for flu, and vaccines may be ineffective against a new mutant strain—and the flu virus is notable for its high rate of mutations. Another great twentieth-century pandemic, AIDS, which has already killed more than 20 million people, illustrates the importance to the spread of a disease of the length of the infectious incubation period. The longer a person is infected and infectious yet either asymptomatic or insufficiently ill to be isolated from the healthy population, the farther the disease will spread before effective measures, such as quarantining, are taken.