While virus particles are really small, the flu and coronavirus are probably mostly spread by droplets, which are quite large. Ever tried breathing mist through a cloth? It gets filtered out just fine.

Secondly, while surgical masks don't fit perfectly, they fit pretty well.
If a mask filters 90% of the air you breath - w/ 10% bypass at the edges - you're still breathing in 90% less droplets. It's likely that a single unlucky droplet is enough to infect you, so that's probably something like 90% less risk of _that_ infection mechanism.
These guys also reported on some actual fit testing, and found that the N95 masks they tested fit just fine out of the box with little effort, with a real world of mostly 95%+ filtration. Doesn't surprise me: if they didn't work, industry would fix them. https://smartairfilters.com/en/blog/coronavirus-pollution-masks-n95-surgical-mask/
Finally, all masks probably remind you not to touch your face.
If you don't want to do your homework, here's a great explanation of how HEPA filters actually work, why they can filter nanoparticles like diesel exhaust just fine (way smaller than viruses), and why even a simple piece of cloth is ~30% effective. https://smartairfilters.com/en/blog/can-hepa-filters-capture-nanoparticles/
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