It’s the dose of inhaled virus

Most transmission on coronavirus is aerosol driven, ie inhaling viruses in the air.

It is not binary - there is a relationship between amounts of particles and severity of the disease.

That makes sense to me because the immune system has good but finite resources to fight invaders.

well researched paper

The amount of dose is dependent on a lot of things, most important of which are 

1. Number of people in the space.  More people, more risk someone is shedding virus.
2. Ventilation (thus outdoor space is much safer than indoor).  Air circulation is good, filtration is better.
3. Length of time spent in the space. Less time , less opportunity to inhale virus.
4. PPE especially  masks. Good masks like N95 reduce the amount of virus escaping from shedders, and the amount inhaled.  Current affairs has an article on the science behind masks

PNAS has a guide  for reducing indoor airborne transmission.

Twitter thread laying out important talking points