The discovery of anthrax cases in Florida will have caused grave concern among security and health officials. It is 100,000 times deadlier than the worst chemical warfare agents known and is easily spread.
Anthrax is not a new disease and has been known since antiquity. It is highly lethal at low doses, however, and because of this is considered useful as a biological warfare agent.
A spore-forming bacterium, Bacillus anthracis, causes the disease. It is so hazardous that there are 100 million lethal doses per gramme of anthrax material, according to the US Department of Defence.
It is found in many plant-eating animals including goats, sheep and cattle, but usually only in developing countries or countries without veterinary public health programmes. The bacterial spores can live in soil for years and under the right conditions can be kept for decades. Given growing conditions they produce fresh bacteria.
There are three routes for infection, through cuts or breaks in the skin, by breathing in anthrax spores - known as "woolsorters" disease - or from eating meat infected with the bacteria.
Infection through the skin initially causes sores, with the infection spreading to other tissues via the blood. It is fatal in up to 20 per cent of untreated cases through this route. Intestinal infection is more dangerous. Symptoms include nausea, vomiting and fever followed by abdominal pain; death occurs in up to 60 per cent of cases. Inhaled anthrax spores represent the greatest risk with nearly all untreated cases ending in death. Initial symptoms are similar to those of a common cold.
There are effective vaccines against anthrax, which are now routinely given to all US military personnel. These protect against the disease through any route.
It is also possible to keep a patient alive with antibiotics such as penicillin, but treatment must begin before the onset of symptoms. Once symptoms begin to occur it is too late to control the disease using antibiotics.
The US Department of Defence describes it as a "silent, invisible killer" that is "easy to weaponise". It is cheap and simple to produce and only requires relatively low high technology to produce.
The spores are very stable and can be stored for long periods as a dry powder. This can be put into munitions or discharged as an aerosol using sprayers. A crop sprayer loaded with anthrax material could distribute large numbers of spores and living bacteria over wide areas. This could cause wholesale infection and many deaths.
A particular problem is that anthrax can't easily be detected. Samples must be collected and then grown in the lab to confirm its presence. Many people could be infected and fall ill before it was known that anthrax was the culprit as symptoms may be delayed for up to a week.
