If you drop anthrax spores in areas where livestock graze or wild animals roam, anthrax's bitter harvest may keep returning. Under the proper weather conditions, spores can regrow and multiply locally. Animals grazing close to the ground may ingest anthrax-infected soil. The animals die suddenly. And hungry humans who butcher, consume or even use the animals' hide or hair are likely to develop anthrax as well.
In nature, luckily, the human disease is less deadly than the animal disease. The vast majority of affected humans develop cutaneous (skin) anthrax. It is slower to develop, and responds nicely to inexpensive antibiotics... if you have the means to get it diagnosed and treated promptly. A wider epidemic can be prevented by vaccinating herds once the disease is recognized.
Zimbabwe's health services have taken an enormous hit lately and are in shambles. People are dying again from the anthrax that was used against them and their cattle 30 years ago, during a long and dirty civil war for majority rule. An FBI informant claimed that Steven Hatfill told friends he had been involved in the original Zimbabwe epidemic. This is one reason he was suspected of having a role in the anthrax letters. Very little is publicly known about who spread Zimbabwe's anthrax, and how it was done.
However, Zimbabwe's experience tells us that even without causing the deadlier (inhalation) form of the disease, anthrax is a terrible problem. Forming a spore that may be viable centuries later, anthrax cannot feasibly be removed from the soil once it lands there.
Zimbabwe teaches that even as we make plans (and spend 50 billion dollars) to mitigate the effects of a biological attack, we should hesitate. We should step back a few paces. Mitigation is a hopeful concept. It may sometimes be effective. But Prevention is guaranteed to work. That is where our focus in the biological arena should be.