Q:

what is this grammar? A plural noun with a singular object

Happy New Year all!
I was travelling in a public transport and saw the passenger notice.
One of the statements on the notice has this information:

Passengers are advised to wear a surgical mask when having symptoms of
  fever.

I am wondering why a singular object, i.e. a surgical mask is used.
It sounds like to me that all passengers are advised to share the same surgical mask when aboard.
Could you explain it please?

A:

A singular object - mask - is used since each passenger is expected to wear a single mask.
If, on the other hand, each passenger was expected to wear multiple items, a plural form would have been used.
For example :

Passengers are advised to wear surgical gloves when having symptoms of fever.

Here each passenger is expected to wear two gloves, so a plural form is used. Of course we can replace "surgical gloves" with "a pair of surgical gloves", which would bring us back to the singular form.

