A very good friend of mine is a PhD epidemiologist at the CDC, he posted this note to a social media site:

Pro Tip 5 - I’ve not seen much about the concept of infectious dose. That is it usually takes more than one microbe to initiate an infection. The number is determined in part by the bug - some like shigella (a bacteria) have a very low infectious dose (around 10 viable bacteria) while others, such as salmonella (another bacteria) have a much higher infectious dose (thousands to tens of thousands). 

I’ve not seen estimates of the ID50 (the number required to infect 50% of those exposed) for SARS-2. While it is generally determined indirectly due to ethical concerns of deliberately exposing humans to pathogens, there may be some estimates in the literature.  These are usually from WAG estimates of natural exposures in defined populations or through studies using lab animals or culture systems.

The infectious dose also varies by individual. Some folks may be exquisitely sensitive while others are much more resistant to infection. A dose that may put one person in the grave may barely raise a fever in another. There are a whole host of factors that play a role here - some have been well-publicized in the media while others remain to be determined.

In some cases, infectious dose has an impact on incubation period (time from exposure to onset of symptoms) as well as severity of illness.  If you are exposed to a big honkin dose - you are likely to get sicker quicker than if you are exposed to a much smaller amount.

These are all rather general concepts and few have been completely resolved for any single bug - let alone a new one like SARS-2.  However, they should make sense to most folks and are hardly rocket science.  

Where this becomes relevant is discussions about short and long periods of close and distant exposure to persons excreting low or high amounts of virus. Mask wearing, crowding, ventilation - all come into play but it all comes down to the seldom mentioned "infectious dose". If you get enough - the bugs win and you lose. If not, you can play another round.