Airborne transmission of disease refers to pathogens that are expelled by an infectious person, and can travel long distances and stay airborne for long time periods.
Whether propelled by sneezing, coughing, talking, splashing, flushing or some other process, aerosols include a range of particle sizes. Those droplets larger than 5-10 millionths of a meter (a micron [µm]; about 1/10 the width of a human hair), fall to the ground within seconds or impact on another surface, without evaporating. The smaller droplets that remain suspended in the air evaporate very quickly (< 1/10 sec in dry air), leaving behind particles consisting of proteins, salts and other things left after the water is removed, including suspended viruses and bacteria. These leftovers, which may be more like a gel, depending on the humidity, are called droplet nuclei. They can remain airborne for hours and, if unimpeded, travel wherever the wind blows them. Coughs, sneezes and toilet flushes generate both droplets and droplet nuclei.
Droplets smaller than 5-10µm almost always dry fast enough to form droplet nuclei without falling to the ground, and it is usual for scientists to refer to these as being in the airborne size range. It is only the droplet nuclei that are capable of riding the air currents through a hospital, shopping center or office building. Diseases like influenza, and rhinovirus are examples of viruses that spread efficiently through the air.
For EVD at least, airborne droplet nuclei are apparently not infectious to primates under natural or near-natural circumstances.
Why that is so is not known, but perhaps it is because this virus does not survive being dried down, or that primates don’t produce enough virus in what is coughed out to make infectious droplet nuclei. To be clear, there may be some EVD in these droplet nuclei - but it has never been shown to cause disease, even when that route has been looked for in the same household as a case of EVD.
Credit to the Virology Down Under blog, by Ian M Mackay