So Brokaw's assistant got the letter on Sept. 18 and didn't test positive until Oct. 12? That's one long incubation period for cutaneous anthrax, 24 days.
From what I have read the symptoms appeared right around 10/1.
She got a rash weeks ago, but didn't have it looked at, until it grew to be the size of a baseball. Then, they didn't release the news, until now.
I do hope that that info satifies your skepicism. : - )