Jonas Salk: What I had confidence in were the results that we had obtained as we went along. We had to understand how to destroy virus infectivity, so that we could do it reliably. Nevertheless, the first time that humans were inoculated it was a matter of some concern. Unknown events might have taken place, things that might have been overlooked. There was some apprehension until that phase of the experiment was over. Before the field trial, I did a test in about 5,000 school children in the city of Pittsburgh which was of the nature to make sure that things did go well, before we went ahead and put this out on a much larger scale. And so, while it is true that we proceed on the basis of things that we know, about which we can have confidence, so to speak, that when you engage in human experimentation, you must proceed in a somewhat cautious manner and be prepared for the unforeseen and the unknowable.