Infections may be winding down in Mexico and causing fewer deaths worldwide than feared, but what is now keeping health officials awake at night is the possibility that a more virulent version will come roaring back and kill millions.
Flu pandemics historically come in waves, often getting worse as they go. The deadly one of 1918, which was also the last H1N1 pandemic, did just that.
One difference between 1918 and now is the possibility of a vaccine. In theory, if this mild outbreak of H1N1 is followed by a lethal wave later in the year, we could make vaccines against what is circulating now and hope they still work against whatever the virus turns into.