People are contagious in the pre-symptomatic or asymptomatic case. So all that's needed is that one employee of the retirement home gets sick and goes to work before they know, and the game is up. Dead patients everywhere.

I'm sure this is discussed and considered on the highest levels of policy in the jurisdictions that have been the most proactive in getting a handle on the epidemic early, and missed in most others.

I'm sure a strict face mask policy could have helped slow things down, but that's sadly been a hotly contested topic.