In reply to stp:
It's not so much about the wearing of face masks, it's more about the process of wearing them, taking them on off etc.
Wearing a mask will mean you touch your face more than you would without one. Therefore if you are not affected and you wear a mask, but get the virus on your hands then adjusting the mask, taking it off etc could transfer the virus to your mouth nose etc.
If you already have the virus then touching your face etc is less of an issue and a mask is likely to reduce the chance of the virus being transmitted through coughing, but not through touching, that risk remains the same, mask or no mask.