A study of ferrets suggests that the flu virus can be passed along before symptoms even pop up. If this is true for people too, and ferrets’ flu reactions are similar to ours, the upshot is that we shouldn’t just refrain from sharing a sniffling friend’s glass of water: even healthy-looking people might expose us to the virus.

The study showed that ferrets could infect fellow ferrets just a day after becoming infected—but after five or six days, the virus was much less likely to be transmitted, which suggests that interacting with flu patients shortly after their symptoms subside could be safe. Until then, wash your hands frequently!

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