If a Martian were to land on earth to study humanity, one of the things that would no doubt surprise him about our race is the pleasure it takes in contemplating its own extinction by various catastrophic means: the crash into earth of a giant asteroid, climate change or the spread of new, virulent and untreatable diseases, especially caused by viruses that emerged from the African jungle.

Of all the viruses to have emerged of late, Ebola is the most frightening. It comes in several varieties of different virulence, with (according to a recent article in the New England Journal of Medicine) death rates from a “low” 40 percent to over 70 percent. Among monkeys the death rate can be 100 percent.

Before Ebola there was Marburg, so named because it was first recognized among laboratory workers in Marburg, Germany. This virus is spread from fruit bats to monkeys to humans, and I happened to be in Rhodesia (as it was then still called) when there was an epidemic there of the disease and 33 percent of the patients died. I remember the reaction in the hospital between panic and pride that it should be in the eye of a world-publicized storm. The question on everyone’s mind was whether it could spread on a large scale from Africa to Europe and North America. Could the virus escape its ecological niche?