Why wear a face mask during this situation on Koh Phangan?
Many Asian people will wear a face mask during this time but also it is normal for them to do in general when they are sick.
This is because they don't want other people to get their germs and get sick too. During this time it is wise to presume or pretend that you are already infected yet showing no symptoms at all but you need to help stop infecting other people who may be more vulnerable.
There are lots of different types of face masks to chose and lots are available for free. Information on caring for them is widely available online.