As America's attention focused on anthrax last week, the federal government reminded doctors to be on guard against other biological weapons, too.
Smallpox, plague and Ebola virus were among the horrors cited in an alert from the Centers for Disease Control and Prevention (CDC).
Here's a quick tour of potential biological weapons other than anthrax that concern CDC the most:
Botulism
The botulinum toxin is the most poisonous substance known, and experts worry that it's also easy to produce and transport. What's more, even the survivors of a botulism attack would need prolonged intensive care, making the prospect of such an attack even more daunting.
Doctors can counter a poisoning with antitoxin, but it has to be given quickly to minimize the symptoms. The antitoxin is available from the federal government through state and local health departments.
Iraq has put the toxin in missiles and bombs. The Japanese cult Aum Shinrikyo dispersed it in aerosols on at least three occasions in Japan in the early 1990s, although the attacks failed.
The toxin, which attacks the nervous system, is produced by a bacterium. It's the same substance that causes about 200 cases of natural botulism every year in the US, such as in food poisoning. One task in recognizing a terrorist attack, in fact, would be distinguishing it from a natural outbreak.
Exposed people would typically begin to feel symptoms within 12 to 72 hours. They would notice blurred vision, difficulty in speaking, droopy eyelids, and muscle weakness that progresses from the shoulders down. Paralysis could make victims stop breathing unless they are put on ventilators, sometimes for weeks or months.
Most victims of natural botulism poisoning eventually recover after weeks or months of care. A recent report found a mortality rate of about 6 percent for food-borne botulism in the US.
While botulinum toxin is highly toxic if swallowed, it isn't very dangerous by inhalation, said David Franz, a bioweapons defense expert at the Southern Research Institute, an affiliate of the University of Alabama in Birmingham.
Plague
The very word inspires fear. In 1994, some 500,000 people fled Surat, India, in fear of a pneumonic plague epidemic.
There is no vaccine to prevent pneumonic plague. But several antibiotics are effective for treatment if given within one day after symptoms appear.
The most common kind of naturally occurring plague is the bubonic form, from the bite of infected fleas. Experts say terrorists would most likely put plague bacteria into an aerosol to be inhaled, causing the pneumonic variety.
The responsible bacterium is widely available around the world. Techniques to make aerosols of the bacterium were developed by the US and the Soviet Union, and Soviet scientists made large amounts of plague organisms suitable for weapons. But the US failed to turn it into a useful weapon because of technical problems, Franz said.
Franz and Philip Carter, a microbiologist at North Carolina State University, said terrorists would face problems in creating an effective aerosol for a mass attack, in part because the floating germ would be short-lived.
Still, others say an aerosol of plague bacteria could be deadly. By one worst-case estimate, just 50kg of bacteria released over a city of 5 million people could cause pneumonic plague in as many as 150,000 people, of whom 36,000 would die.
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