The world has a lot of dams-45,000 large ones, according to the World Energy Council, and many more at small scales. Its hydroelectric power plants have a generating capacity of 800 gigawatts, and they currently supply almost one-fifth of the electricity consumed worldwide. As a source of electricity, dams are second only to fossil fuels, and generate 10 times more power than geothermal, solar and wind power combined. With a claimed full capacity of 18 gigawatts, the Three Gorges dam in China can generate more or less twice as much power as all the world's solar cells. An additional 120 gigawatts of capacity is under development. One reason for hydropower's success is that it is a widespread resource-160 countries use hydropower to some extent. In several countries hydropower is the largest contributor to grid electricity-it is not uncommon in developing countries for a large dam to be the main generating source. Nevertheless, it is in large industrialized nations that have big rivers that hydroelectricity is shown in its most dramatic aspect. Brazil, Canada, China, Russia and the United States currently produce more than half of the world's hydropower.

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Estimates of COVID-19's case fatality rate-the proportion of infected people who die-suggest that the coronavirus is less deadly than the pathogens behind other large-scale outbreaks, such as those of SARS (severe acute respiratory syndrome), MERS (Middle East respiratory syndrome) and Ebola. But it seems to spread more easily. Calculations of the virus's 'basic reproduction number' (R 0) suggest that each infected person will pass the virus to an average of 2-2.5 people. Like the case fatality rate, R 0 is an estimate that varies considerably, and is likely to be revised. 0 2 1 3 5 4

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Researchers are redoubling efforts to understand links between biodiversity and emerging diseases-and to use that information to predict and stop future outbreaks.