A disease doesn’t count as a pandemic until it spreads worldwide – Ebola killed more than 11,000 people across West Africa before it was brought under control, and that was just an epidemic. The most modern pandemics include the Spanish influenza, circa 1918 (as many as 50 million killed), and HIV/AIDS (35 million dead).

As Chinese officials attempt to stem the latest bird flu outbreak, global public health officials are racing to get ahead of what they call the next “big one”: a disease that will kill tens of millions. It’s all about preparedness, and a large part of that is spotting outbreaks early, so action can be taken to contain any situation before it spirals out of control.

It’s anyone’s guess when and where the next major epidemic – or pandemic – might emerge. Crimean-Congo haemorrhagic fever, Nipah virus, and Middle East Respiratory Syndrome (MERS) are the likely.