In recent years, in places from Mexico to Asia, outbreaks in flu cases have been matched with outbreaks in people wearing surgical masks. Those wearing masks have at least a degree of protection against the flu.
Many people are under the misconception that these masks are worn by doctors to protect themselves from patients, but it is not the true. Surgical masks are designed to prevent spittle, mucus etc. from falling from the doctor into the open wound of the patient in surgery.
For the wearer of a mask to be protected from the flu, the mask must seal to the face and the holes must be smaller than the virus. Surgical masks do not even conform to the face and offer various openings for a virus to pass through.
Furthermore, the flu virus can easily pass through the mask since the holes in the weaving are much larger than the virus. You can find more about full face masks via https://designermasks.com/ or various other online sources.
Even expensive, quality masks such as the N-95, which can filter out 95% of particles down to.3 microns (and a human hair is about 100 microns in diameter) are not always effective in stopping viruses.
Besides giving the user a false sense of security, surgery masks can be counterproductive in other ways. By keeping the face warm and moist, many mask wearers may be creating conditions that help the virus survive and reproduce. The user may also be further exposed to the virus when the mask is removed and disposed of.
The one situation when a mask may help is when the wearer already has the flu. By covering the mouth, masks can help prevent an infected person from spreading the virus to others..