How many is "so many"? Do you have a source for this information?
1 is too many but there were more than 1. MSF doctors as well as the early infections of American charity physicians who were taking full barrier precautions. (Foreign doctors).
In fact, this is the first time so many health care practitioners have been infected with ebola compared to previous outbreaks.
IMO,the virus has mutated to make this possible, perhaps by not producing symptoms in a person yet allowing them to be infectious for a longer period before they receive a diagnosis that they are infected. The CDC guy today was assuring everyone at today's news conference that even if people were infected with Ebola, they wouldn't be infectious unless they were showing symptoms. I think they just said that to keep people calm in the USA as well as the people who shared the plane with the guy on the way over.
Also, many African health care workers were infected who may have not been in hazmat suits, but were wearing masks, gloves, and googles, which are traditional barrier protection in hospitals and clinics even in North America You can see how this outbreak was actually launched by health care workers who spread it to other villages in this NEJM graphic: