This is the same as at colleges. Cases just mean that it is very contagious. So is perhaps a form of a common cold that goes through a classroom.... in the old days. The diffference is that people don't get tested for other diseases with a 99.7 survival rate unless it is making them very sick with symptoms.
Actually It is reported that "A new study says asymptomatic coronavirus spread might far more rare than most experts believe — a new study says the more severe the illness, the more likely it is for a COVID-19 patient to infect others.