Why was the little girl (bless her heart) throwing up? Norovirus, perhaps? Not saying that is the case, but we all know the symptoms. Unless they were certain it was not, the parents made a deliberate choice to expose everyone on that flight and contaminate the airplane.
Last year, some of my extended family members came to a family gathering a couple of days after the "stomach flu" had gone through their household. We all got it.
Sharing is not always caring.