The past month has brought us a choice of potential global disasters. Maybe the Black Death will return, and kill more than half the people on the planet as it once killed over half the population of Europe. Or we could have another meteor strike like the one that hit Antarctica 780 000 years ago, which could do much worse than that. It has become orthodox to link the great extinctions of the archaeological record to huge asteroid strikes. At the International Geophysical Congress in Glasgow on 18 August, Dr Frans van der Hoeven of Delft University of Technology in the Netherlands revealed that a similar asteroid hit Antarctica only 780 000 years ago, a mere blink of an eye in geological time. We are still here for three reasons, all of them flukes. The asteroid broke up just before hitting the earth, creating five smaller impact craters over an area measuring 1 300 by 2 400 miles (2 100 by 3 800 km) rather than a single huge impact crater. Most of the pieces melted through the deep East Antarctic icecap before cratering the underlying bedrock, which limited the amount of dust boosted into the atmosphere. And there was already permanent winter over most of the planet, so it was much less of a shock to the biosphere. Asteroids got the dinosaurs, but they didn't get us. Whereas the Black Death did get Europe repeatedly between the fourteenth and the seventeenth centuries. Then it went away ? but maybe not for good. For the past century, the Black Death has been explained as an outbreak of bubonic plague, spread by fleas living on rats, but there have always been huge problems with that diagnosis. The fatality rate is far lower with bubonic plague, the incubation period doesn't match, and the standard measure to contain the spread of the Black Death ? quarantine ? would not have worked if the vector was rats. Now along comes "Return of the Black Death," a book by Christopher Duncan, Emeritus Professor of Zoology at Liverpool University, and social historian Susan Scott, which convincingly argues that the Black Death was a haemorrhagic fever related to Ebola and other current quick-killer African diseases that spread directly from person to person. What made it so lethal was its extremely long incubation period, almost a month. That allowed plenty of time for an affected person to infect many others before the symptoms appeared. It eventually died out in Europe because by the seventeenth century a large portion of the surviving European population had developed a genetic immunity to the virus. Scott and Duncan suggest that the immunity of between 5 and 20 percent of Europeans to the HIV virus is a relic, now diluted by time, to this tragically acquired immunity to the Black Death. But they also suggest that the original virus or some mutated successor is still out there somewhere ? and that by now Europeans' immunity is severely eroded, while nobody else has any at all. Meanwhile, in Vietnam, three people have died in the past month from the A(H5N1) avian influenza virus, which has a two-thirds mortality rate in human beings. "We're talking hundreds of millions of people afflicted if it is a pandemic out there lurking," said Anton Rychener, Vietnam director for the UN Food and Agriculture Organisation. "What I have learned makes me shudder." If one thing doesn't get you, another thing will. Gather ye rosebuds while ye may.