There's a lot of flu flying around just now, and there's no way of telling if you're sharing your bus/office/home with someone with one of the infected - until it's too late.
Well, actually, academic researchers say there is a way - just by looking at them.
John Axelsson of the Karolinska Institute, who co-wrote a study in the journal Proceedings of the Royal Society B, says we can detect "subtle cues related to the skin, eyes and mouth" which let us know people are ill.
The study injected 16 paid volunteers - eight men and eight women - with a molecule found in bacterial membranes called lipopolysaccharide.
Don't worry, it's not harmful.
Axelsson explains that people do not really become sick from the bacteria, but their bodies are tricked into activating their immune systems - complete with feelings of sickness.
The subjects were photographed two hours and 10 minutes after the injection - when they claimed to feel unwell - and also snapped them on a different day after giving them a placebo injection of harmless saline solution.
Then 60 students from universities in Stockholm were given the photos to look at for five seconds and asked if the person in it was ill or healthy.
Of the 1,215 images identified as “sick,” nearly 800 were correct conclusions - which they say is more than simply probability.
So what do you look out for?
Well, another group of 60 students rated the following telltale features:
- pale lips
- droopy mouths
- change of skin colour
- heavy eyelids