It only takes one person. The vast majority of the Hutterite cases can be traced back to two people. You can take all the precautions you want, but all it takes is one person deciding to ignore them once or twice, especially attending large gatherings or not staying home when sick, and there you have it.
And I'm sure this happens every year with cold and flu, but now it is being documented. I just hope we are learning the lessons - they are not just applicable to Covid 19.