This is Case 18 by UCLA's reckoning. The patient is a bookkeeper and thus handles quite a bit of mail; the presumption is that she tore open a piece of mail cross-contaminated by the 10/9 (Leahy and Daschle) letters at the Hamilton Township processing facility where four postal workers were infected.
I thought that anthrax spores passed through envelope paper like sand through a sieve, but apparently the paper acts more like a sponge. Spores were absorbed by the Leahy and Daschle envelopes, then were squeezed out by the postal handling machinery. Other envelopes absorbed these spores in turn; when those cross-contaiminated envelopes were ripped, the spores along the tear became aerosolized. For Case 18, that meant contracting cutaneous anthrax through a small break in her facial skin (such as a pimple); for poor Otillie Lundgren, who habitually ripped her junk mail in half, that meant dying of inhalation anthrax. So the lesson for today, kids, is to use a letter opener, the sharper the better.