As a weapon of mass destruction, anthrax has so far proved to be less than effective - with just one fatality and a handful of illnesses resulting from the spate of recent cases. But as a form of psychological warfare, it's having a profound impact.

Discoveries of additional cases - including a letter containing anthrax sent to Senate majority leader Tom Daschle's office, and the 7-month-old son of an ABC News producer testing positive for exposure - have undoubtedly added a new level of gravity to the situation, which the government now officially refers to as terrorism.

Yet, with these new cases have also come literally hundreds of hoaxes and false alarms, demonstrating that terrorism doesn't have to involve widespread loss of life to be effective. While not nearly as devastating as the Sept. 11 attacks, the anthrax cases have been arguably more powerful in getting under Americans' skin, creating a sense of fear that has disrupted daily routines.

Newsrooms have been evacuated after receiving letters laced with talcum powder, and jittery Americans across the nation have called authorities to test everything from beach sand to confetti.

Congress, seems especially on edge. Eleven offices near Senator Daschle's office have been shut down, and all mail to the House and Senate has been suspended. Even before the letter to Daschle was discovered, Sen. Dianne Feinstein (D) of California was already instructing everyone on her staff to wear masks and gloves while opening mail, and Senate minority leader Trent Lott has called for bulletproof glass to protect the Senate floor.

So far, public-health officials have managed to respond to each case relatively quickly. But experts warn they may be increasingly hindered by the flood of calls and other incidents that continue to demand their attention.

"This is the kind of thing that terrorists want - they want to put sand in the gears," says Rep. Christopher Shays (R) of Connecticut. "They don't always need to do something dramatic. But this forces us to spend a lot of time and attention on something that's fairly easy to do. You know, just get some anthrax and contaminate a letter and send it to whomever. Everybody becomes a potential target, at random."

Indeed, it's this sense of randomness, and an innate fear of the unknown, that has managed to deeply unsettle so many Americans.

"What makes [anthrax] an effective terror weapon is that the psychological impact is really profound," says Jessica Stern, a lecturer on terrorism at the Kennedy School of Government at Harvard University in Cambridge, Mass. "People have a natural, age-old aversion to poison. And so I think it's completely understandable and natural that people respond this way."

Ms. Stern says psychologists have identified 18 variables associated with biological weapons that tend to create a disproportionate sense of threat. These include: invisibility, a lack of scientific understanding of the substance, the possibility of long-term effects, and not knowing immediately when you've been affected.

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