Natural epidemics have the power to terrify but even more chilling to some is the idea of diseases as weapons of war.
The use of deadly germs to sow the seeds of terror is not a new idea. History books contain well-documented cases: medieval armies catapulting rotting corpses over castle walls; blankets from smallpox patients, given as peace offerings to Native Americans.
Little has changed in the 21st century except for the ability to infect more people, with deadlier results. The Centers for Disease Control, the federally-funded organization that deals with both bio-terrorism and epidemics, says the most deadly germs are the so-called Category A pathogens such as smallpox and anthrax.
Dr. William Raub, a U.S. Public Health Emergency official says although it was eradicated in the late 1970's, smallpox still poses a threat.
"The only stocks of the virus we know about are in two high security well-contained facilities in the United States and Russia. But there is the worry that there might be clandestine sources of that virus in other parts of the world that could be used by terrorists."
The power of anthrax to terrorize was demonstrated just one week after the September 11th attacks of 2001, when letters containing anthrax spores were mailed to the offices of U.S. lawmakers and media outlets on the U.S. east coast.
Dr. Raub says anyone who came into direct contact with the anthrax tainted letters would have experienced the first symptoms within 48 hours.
"This is a very unforgiving organism and when those symptoms appear, typically a dry cough, death within 36 hours is highly likely, about a 90% percent fatality rate untreated and even with treatment at that stage, 50 percent at best."
By the end of 2001, five people had died from anthrax inhalation, and 18 others were infected. And suddenly, millions of people were afraid to open their mail. The reason for the attacks and who may have been responsible for them are still unknown but scientists quickly learned that disseminating information quickly and accurately was almost as important as finding an antidote.
"Probably the single biggest lesson was communication, helping the public understand the nature of the threat."
Today, bio-defense research is yielding more effective vaccines that have fewer side effects and new diagnostic tools are being developed that can identify pathogens with greater accuracy.
Despite the advances, Dr. Anthony Fauci, who heads the biodefense program at the National Institutes of Health, says it's impossible to develop countermeasures against every possible threat.
"I don't think there will ever be a time when you can sit back and say OK, now we're completely protected against any possibility of a bio terror attack. That's just not in the cards, cause that's not the nature of the problem. It's a forever moving target."
Bio-terrorism is also difficult to carry out. Scientists say an attack that would cause massive casualties, is more likely to come from conventional weapons or chemicals. In the 1980's, Saddam Hussein used poison gas to kill thousands of Kurds in Northern Iraq.
And in 1995, the apocalyptic religious sect, Aum Shinrikyo, released Sarin gas in a Tokyo subway killing 12 commuters and injuring thousands. Dr. Ken Dretchen, a bio terrorism expert at Georgetown University in Washington, says the availability of chemical agents makes them a more viable alternative for extremists.
"It's the older chemicals that we still have around and we have a lot of these things stockpiled in the United States, many of them in military bases. But the fact is these are the things which we worry more about, because one - the immediacy and second of all, the availability, and they're already here."
Dr. Dretchen, who developed a portable chemical antidote injector for the U.S. military, says any biological or chemical agent that can cause devastation can be exploited but he says turning such agents into weapons of mass destruction also requires a great deal of scientific knowledge and resources. Chemical and biological terror attacks against civilian populations remain a very real threat but scientists say, so are natural disasters.
"In many respects, mother nature is the worst bio terrorist. If we have a pandemic influenza attack in the sense of the natural evolution with no bio-terror involved, just the natural evolution of a pandemic flu such as what we're concerned with which is going on in Asia and now spreading into Russia and Siberia; if that happens and we a get a serious pandemic flu, that could be worse than any deliberately engineered or deliberately propagated attack."
Both the Centers for Disease Control and the National Institutes of Health believe the world is better prepared for a bio terror attack than it was four years ago.
But there is consensus that more international cooperation is needed to develop an early warning detection system for both chemical and biological threats, including new strains of influenza that could be exploited by terrorists.