Every few weeks, it seems, an asteroid approaches close enough to the Earth that people worry about an extinction-level impact. Ditto with nuclear war; if it's not Iran, it's North Korea, or Donald Trump. End of the world as we know it, right?

Well, yes, but maybe we have something even scarier to worry about.

In a 2015 TED Talk, Bill Gates explained that the biggest threat to human life today isn't asteroids or nuclear bombs but something smaller and more insidious: some not-yet-evolved strain of the common flu.

His warning then seems prescient this year, because the vaccine for the current flu season proved ineffective, resulting in more deaths that usual. That's worrying because a lethal strain of flu can kill millions, as it did in the 1918 pandemic.

Yes, today we have flu shots, but flu evolves more quickly than bacterial diseases. While scientists create a vaccine for each year's flu season, they guesstimate which strain will spread, making each year's vaccine hit or miss.  Even then, flu vaccines are never 100% effective.

How big is the potential threat? According to a recent article in The Atlantic, while a nuclear bomb dropped on New York City might kill 10 million people, experts say a flu pandemic might kill as many as 33 million people in 250 days.

As usual, Gates is putting his money where his mouth is, by launching a $12 million challenge to scientists and researchers to find a universal flu vaccine. Absent such a vaccine, though, a widely lethal flu pandemic may currently be inevitable.

While the world hasn't suffered a pandemic for, well, for centuries, previous pandemics (like the 14th century plague) were horrible nearly beyond imagination, with death rates of up to 90%. There are areas of France that still haven't recovered.

So let's hope that Gates can be as effective at eradicating flu as he's been in reducing deaths from malaria. Otherwise we might end up paraphrasing T.S. Elliot and discover that "This is how the world ends, not with a bang but with a sniffle."