For thousands of years, Anthrax was known as a livestock pest, regularly killing animals who grazed on land infested with Anthrax spores (where the animals breathed in the spores as they pulled up grass and released the spores from the soil.) Humans could get infected as well, usually by getting spores on a cut. This skin (cutaneous) form of Anthrax was fatal in up to 20 percent of the victims, depending on how potent the Anthrax strain was and how many spores got into the sore. People who worked with sheep's wool also got cutaneous Anthrax, did those working with the hides of animals who grazed in areas containing Anthrax. In the 1970s, imported wool from an Anthrax area, improperly cleaned, infected a number of Americans. Anthrax is a bacteria, and some people and animals can fight off infections and even develop an immunity.
Anthrax has long been pitched as an effective bio-warfare weapon. Britain developed a military form of Anthrax during World War II. At the time, it was seen as an effective weapon, because the Germans didn't have antibiotics (only the Allies had this then-new medication that easily cured Anthrax infections.) Since then, work has continued on Anthrax, developing more potent strains (so less of it was needed to kill) and making Anthrax resistant to antibiotics (difficult to do, although current genetic engineering techniques make this easier to do if you have the qualified scientists and engineers.) . The major problem with Anthrax is delivering it. The spores, in their natural form, don't travel well in the air. "Militarizing" Anthrax consists of processing the spores so they don't clump together and thus can more easily float away in a breeze. But sunlight and heat can kill the spores, and even if they float through the air, they can disperse so that anyone breathing them in will not get a fatal dose (10,000 to 50,000 spores.) Thus the need for militarized Anthrax to be grown from more powerful strains.
Naturally occurring Anthrax (which exists in most parts of the world) varies in its potency. Wealthier nations, like the United States, give animals in Anthrax ridden areas a vaccine that protects them. There have long been vaccines for humans as well, to protect farmers and veterinarians. Agricultural researchers have collected many strains of Anthrax, and the more potent ones are kept and cultured to provide material to test new vaccines. But even the most potent militarized Anthrax isn't that powerful. We know this from an military Anthrax accident in 1979. A Russian biological warfare plant outside the city of Sverdlosk accidentally released some militarized Anthrax. Thousands of people were in the area and were infected. But fewer than a hundred died. What was particularly discouraging to Russian military bioweapon scientists was that only one of the dead was of military age, and he was already ill from other ailments. All of those that died from the Anthrax were old, and usually sick. All the victims had weakened immune systems. Many had lung ailments. The Russians initially denied that there was an accident, and did not treat the locals for Anthrax. Later they said the deaths were caused by people eating meant infected with Anthrax (a common way for people to die from Anthrax.) It was only after the Soviet Union fell apart that Western researchers were able to get into the area and interview survivors and discover that people with normal immune systems were able to fight off an Anthrax infection.
The 2001 Anthrax attacks in the United States, delivered by letter, killed one and infected less than a dozen others. A form of natural Anthrax was used. More will die and get ill, but not from Anthrax. Millions of people are taking powerful antibiotics just in case they were infected. This massive use of antibiotics will cause other bacteria to become resistant to antibiotics and the resulting "super bugs" will kill a lot of people (a trend that has been noted over the last decade or so.) The problem with Anthrax as a weapon is that you have to use it in secret, and get a lot of people to breathe in the spores. While the less lethal cutaneous form announces itself with a ugly sore (which can then be treated with antibiotics), the pulmonary (breathed in) form announces itself with flu like symptoms a few days after the infection. By then, it is too late and death almost always follows. But if you know you have breathed it in (and a test can confirm this), you can be treated with antibiotics. So far, Anthrax has not really made the jump from livestock pest to biological warfare weapon.