Infections can be caused by yeasts/fungi, protozoa, bacteria and viruses in reducing order of size and with some overlap. If the means of infection is airborne, then we can try and protect ourselves from infection by wearing a respirator or mask to filter out the airborne particles. You can get an idea of the size of fungal spores by placing a ripe (open) mushroom on a piece of white paper. You should see the brown 'staining' from spores falling out of the mushroom gills. You may be able to see some fungal spores with a good magnifying glass, but you'll need a microscope to view protozoa or bacteria and an electron microscope to view a virus.
Here's a quick overview on viruses and bacteria and how diseases caused by them are transmitted:
This Pet Education site goes into more detail and is particularly well worth reading if you have pets:
How Fast Can a Virus Spread? Faster Than You Think!
Now you know why you should regularly wash your hands and keep your hands away from your eyes, nose and mouth!
So how does the Flu spread?
"Most experts think that flu viruses are spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Less often, a person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth or nose."
And here's another article from the Centres for Disease Control and Prevention that determined the size of those droplets and how long they remained in the air:
"General Description: Coughing, sneezing, talking, and breathing generate an aerosol of airborne particles with diameters that can range from a few millimeters to less than 1 µm. This study investigated the potential for influenza virus to be carried by small particles. Researchers at NIOSH developed a two-stage cyclone bioaerosol sampler that can collect air samples and separate airborne particles into three size fractions (greater than 4 µm, 1-4 µm, and less than 1 µm). Attenuated influenza virus was aerosolized in a laboratory calm-air settling chamber and airborne particles collected with the NIOSH bioaerosol sampler were assayed for the presence of influenza virus.
Relevance to Worker Safety and Health: Particles less than 10 µm in diameter can remain airborne for hours and are easily inhaled deeply into the respiratory tract."
Do N95 respirators provide any protection against airborne viruses, and how adequate are surgical masks?
CONCLUSION: "The N95 filtering face piece respirators may not provide the expected protection level against small virions. Some surgical masks may let a significant fraction of airborne viruses penetrate through their filters, providing very low protection against aerosolized infectious agents in the size range of 10 to 80 nm. It should be noted that the surgical masks are primarily designed to protect the environment from the wearer, whereas the respirators are supposed to protect the wearer from the environment."
Note that the conclusion presumes that the airborne viruses are free floating, not included in expelled droplets (aerosolized). But what happens if the virus is aerosolized?
Viable viral efficiency of N95 and P100 respirator filters at constant and cyclic flow. (Note that this study is for better fitting respirator filters, not masks)
"The filtration efficiency of selected NIOSH-approved particulate N95 and P100 filtering facepiece respirators (FFRs) and filter cartridges was investigated against the viable MS2 virus, a non-pathogenic bacteriophage, aerosolized from a liquid suspension. Tests were performed under two cyclic flow conditions (minute volumes of 85 and 135 L/min) and two constant flow rates (85 and 270 L/min). The mean penetrations of viable MS2 through the N95 and P100 FFRs/cartridges were typically less than 2 and 0.03%, respectively, under all flow conditions. All N95 and P100 FFR and cartridge models assessed in this study, therefore, met or exceeded their respective efficiency ratings of 95 and 99.97% against the viable MS2 test aerosol, even under the very high flow conditions. These NIOSH-approved FFRs and particulate respirators equipped with these cartridges can be anticipated to achieve expected levels of protection (consistent with their assigned protection factor) against airborne viral agents, provided that they are properly selected, fitted, worn, and maintained."(my emphasis)
Given we feel strange enough wearing a mask in public, I doubt many of us would want to wear a respirator, as we'd probably cause a public panic! Whether its a mask or respirator, properly fitting a facial/procedure mask is the most important step. I'm amazed at how often I see people wearing masks in medical settings when they haven't even squeezed the soft metal insert so that there are no gaps around their nose. There's not much purpose in wearing a mask if you don't actually filter the air through it!
In summary, when you are talking, coughing or sneezing, you expel fine droplets of moisture that may contain virus particles.* A mask easily captures the moist droplets, but may let virus particles through, because they are about one hundredth of the size of a bacterium and can fit through the pores in a mask. That's why it is more effective for sick people than for well people around them to wear masks when trying to prevent the spread of infection. Likewise I'd say that's why you should regularly replace masks; if the mask is left to dry out and then reused, you could be blowing out/breathing in the virus particles captured earlier after the moisture has evaporated, freeing the virus particle.
* I've witnessed this personally and it is fairly easy to verify. Just enter a very dark room with a torch that has a good direct beam and shine it across the front of your mouth and talk. You should see a fine spray of moisture droplets coming out of your mouth and slowly drifting towards the floor.
Photo: Not all bugs are bad. Bee departing a zucchini/courgette flower, covered in pollen (something else a facial mask may help you with if you get bad hay fever...)