I am inclined to believe that this is 1) Getting the disease from actual symptoms (coughing) and 2) Surfaces. and 3) There’s some kind of predisposition factor (s) such as blood types, tissue types, comorbidities, involved.
I did a quick calculation when I realized that my illness was due to SARS-CoV-2.
The result of JUST my wife & I being the sole infected at our respective places of employment that we potentially exposed about 300-500 people directly. Indirectly the potential exposure was in the thousands (this was 2 weeks before any “lockdown”). Based upon the demographics of people we work with, there “should” have been DOZENS of ICU admissions at a minimum; the reality is that hospital staff got furloughed due to lack of traffic in the 3 week window overlapping our illnesses (for which we were BOTH asymptomatic until onset of fever).