There are two main face masks used in medicine:
The first is the well-known rectangular “surgical mask”, which is the most common style of mask worn by the general population. The design has not changed in over one hundred years. If you look at photos from the Influenza 1918 pandemic, you’ll see the same masks as those being used today in the worldwide COVID-19 epidemic – 102 years later.
These masks were never designed to cope with pathogens the size of viruses. They were designed to block surgeons from expelling large droplets onto the surgical field, while likewise also protecting the surgeon from fluids.
The CDC has recently concluded there is no difference in wearing, versus not wearing, a typical surgical mask as protection against viral sized aerosol pathogens. However, surgical masks do have a role in protecting the public from individuals who, themselves, are spreading viral pathogens.
In contrast, a “Filtering Facepiece Respirator Mask”, the most common of which is the N95, is composed of filter material capable of protecting the wearer to a much greater extent than a simple surgical mask.