I've wondered this too. I can only assume it's because they send infected patients home, and they spread it to their families. That and hospitals & nursing homes, where infected staff may be spreading it between the patients/residents?
Pitman covers this in his article: communication back then between Europe and the US was much more difficult. Sending UUCP emails meant at least a 48hr round trip and so the US committee basically couldn't be bothered cooperating with their European counterparts. The Europeans - at least as Pitman imagines it - got upset and did their own thing