first, there are two types of masks
- those folded, flat masks
- formed, form fitting N-95 masks
The folded flat masks are just for keeping your spittle off of other people.
They do not help for anything else.
The N-95 (keeps out 95% of particulate dust) can be a barrier to you breathing in viruses and bacteria. So, they are better protection than nothing.
However, over time, they get moist from your breath and can be a way of accumulating, and keeping alive the viruses you want to avoid.
Further, when you are most virulent, is just before you have any symptoms. That is when you should be wearing a mask to protect other people, but you don't know to do that.
When a person is sick and having trouble breathing because their lungs are filling up with mucus, a mask can kill them, just like putting a pillow over their face.