Apparently it actually is a matter of the size of the dose. I think it is a misconception a lot of us had early on that any contact at all with the virus would cause infection. Note, we still don’t know what the critical dose is, but small doses should be fought off by a healthy immune system.

An article from the NYT discussing this.

https://www.nytimes.com/2020/05/29/health/coronavirus-transm...