[Southack]

"If they filled the envelopes in a public park, then down wind, or in the soil, there would be left over spores. Someone would eventually get an infection."

No.

You have random anthrax spores in wool processing facilities, as well as on the ground in sheep-grazing areas...without people getting infections.

An anthrax infection takes hundreds of spores at a single moment. It's a weak bio-agent.

It's the first thing for which a vaccine was ever developed (Louis Pasture circa 1850).

[Battle Axe]

You are right that it takes a "bolus" of spores to contract an infection under normal conditions. You just about have to open the bottle and pour it into a sore!!!

However, they did feel that perhaps 3 spores into the lungs would cause an infection. Our bodies are counting on our nose hairs and mucus to catch them before the lungs.

However, I think you are thinking of smallpox as being one of the first vaccines........correct me if I am wrong.

Smallpox to cowpox

[muawiyah]

Or, if you are 90 years old, it can take only a single spore. But you have to breath deep.