1) It looks like a normal cold when it starts.
2) You feel better for a few days.
3) By the time the second round of fever hits, it's too late for antibiotics.
4) Relatively easy to gather means all the difference to low-tech operations. They don't have to have a 100% success rate, or even a 20% success rate. 10% success at a University of Michigan football game means more people with anthrax than died in the WTC disaster.

Horrible thought: the speculation now is that the anthrax may have come in a letter.

Can you say, "Publisher's Clearing House"? How about a mass mailing from "looks sufficiently legit for millions of people to see what they 'may have won'"?

Smallpox: They stopped mandatory vaccinations in 1970 IIRC. I have a vacc scar, but my son doesn't.