Natural epidemics like the Ebola virus or SARS have the power to terrify, but even more chilling is the idea of disease used as a weapon. An attack of anthrax through the U.S. postal system in 2001 put the United States on high alert to the problem of biological terrorism.
The use of deadly germs to terrorize is not new. Medieval armies catapulted rotting corpses over castle walls. European explorers gave the blankets of smallpox patients as peace offerings to tribes of native Americans.
Disease remains a weapon in the 21st century. The anthrax powder distributed through the U.S. mail four years ago killed five people and infected as many as two-dozen others. The official overseeing U.S. emergency health preparedness, William Raub, says the government considers anthrax one of the two leading biological weapons for which the world must prepare. The other is smallpox.
"A terrorist using the smallpox virus or using the anthrax organism could create mass casualties in a public health catastrophe," he said. "All of our efforts in developing new smallpox vaccine, developing new anthrax vaccine, building a national stockpile with literally over a billion antibiotics are all part of how seriously we take those two threats."
Behind this preparation is the fear that there might be clandestine sources of smallpox, despite its eradication in the 1970s and the storage of remaining stocks in high security vaults in the United States and Russia.
The physician who directs the U.S. government's infectious disease research, Anthony Fauci, says the United States is not the only potential bioterrorist target.
"Obviously the United States and the U.K., because we're being targeted by extremists for a variety of reasons related to Middle East activities, are vulnerable, but I think everyone and anyone throughout the world is vulnerable to bioterror attacks," he added.
Besides the work it is doing on experimental smallpox and anthrax vaccines, Dr. Fauci's agency is testing the first vaccine for Ebola virus, considered another potential top biological agent. It is also preparing for a new generation of antibiotics, antiviral medicines, and other vaccines.
In addition, Washington has increased financial assistance to U.S. state health departments and emergency agencies, which would be the first to respond to an attack. Dr. Fauci says it is also working with other countries.
"Starting just a few years ago right after the anthrax attack, we have been in very close collaboration with many of our allies -- Canada, the U.K., Australia, several European countries -- exchanging information, trying to coordinate what a response would be, but also sharing scientific information," he exlplained.
Besides smallpox, anthrax, and Ebola virus, public health officials say many other biological agents can serve a terrorist's purpose. For example, they worry that terrorists could poison the food supply with salmonella or E-coli bacteria.
But some experts believe a greater threat would be terrorist use of toxic chemical agents. An incident occurred in 1995 when the Japanese religious sect Aum Shinrikyo released sarin gas in the Tokyo subway system, killing 12 commuters and injuring thousands. Georgetown University drug expert Ken Dretchen says chemicals are easily available, take far less expertise to make into weapons than microbes do, and make people sick much faster.
"The chemicals are things that have been around for a long, long time and have actually been utilized going all the way back to World War I. The older chemicals that we still have stockpiled in the United States, many of them on military bases, these are the things that I think we worry more about because, one, the immediacy and second of all, the availability," he said.
Mr. Dretchen, who developed a portable chemical antidote injector for the U.S. military, says terrorist biological or chemical attacks are not as big a threat as natural disasters and diseases, which cause far more deaths and injuries. But if terrorists act in this way, he is confident in the government's growing ability to respond.
"I mean, I don't go to sleep at night worrying tomorrow whether or not there is going to be a chemical or a biological attack," he added.
Anthony Fauci, who is paid to worry about such things, says that if a biological attack never occurs, the money spent preparing for one is not wasted.
"We put it under the larger umbrella of emerging and re-emerging infectious diseases. In some respects, if we look at it as all one, it's easier because you need a fundamental level of preparedness that spans deliberately and naturally occurring threats," he noted.
With avian influenza looming, together with SARS, AIDS, tuberculosis and more than 1,400 other infectious diseases known to medicine, there will always be a health enemy to fight.