conclusion that it was because they weren't masking, or there was some masking failure around them.
We would have to delve into how they contracted the virus.
You might say, if we can prevent ONE illness by having everyone mask, then it is worth it. This is similar to leaving the ninety and nine to find the one lost sheep, which is definitely a valid argument.
The likely truth is that any measures we take to prevent this virus from spreading are likely to be minimally effective. We haven't found the answer in 3 months of trying to "do something about this". At this point it should seem obvious to everyone that the virus is going to make its way through our population and we should be prepared when it hits us.