Should people wear a mask in public if they feel unwell with flu?

Masks can reduce the number of particles containing viruses that are released from the mouth and nose of someone who with a respiratory infection. Photograph: Peter Titmuss/Alamy

An NHS leader has said people who are unwell with flu “must” wear a mask in public. A UK government official has said if you need to go out while sick, you should only “consider” wearing a mask.

So what is the official advice, how serious is the threat posed by flu, and what should you do if have symptoms?