But when the outbreak is caused by criminals, as is the situation in the anthrax crisis, a decline in cases has little meaning.

No new anthrax cases have been reported in more than a week, but public health and law enforcement officials know that more cases could crop up any time.

That is the specter hanging over officials as they work around the clock to determine the source of the anthrax outbreak, which FBI officials -- and President Bush -- say was caused by still-unidentified terrorists.

"This [crisis] will not be over until the criminals are caught," says Dr. Julie Gerberding of the U.S. Centers for Disease Control and Prevention in Atlanta.

However, more than a month after the first case of anthrax was identified in a tabloid photo editor in Florida, authorities acknowledge they have few leads in tracking the source of anthrax spores sent through the mail or possibly spread by other means.

As of Friday, 17 confirmed cases and five suspected cases of anthrax had been identified since the outbreak began in early October. Of the confirmed cases, 10 are inhalational anthrax, the deadly form of the disease. Four victims have died. The other cases are skin anthrax, from which patients usually recover.

A senior FBI counterterrorism official acknowledged during Senate testimony last week that, one month into the investigation, his agency still cannot answer such basic questions as how many laboratories in the United States handle anthrax bacteria.

Sen. Dianne Feinstein (D-Calif.) harshly criticized the agency for knowing so little about who had access to the strain of anthrax sent through the mails. "It's a real national security risk to our people to leave such an open door to the movement, possession and handling of these agents," she said.

CDC at center stage

Anthrax has brought sharp focus on the other lead federal agency in the bioterrorism battle -- the CDC, headquartered near Emory University in Atlanta.

Hours after the first case was confirmed Oct. 4 involving photo editor Bob Stevens of Lantana, Fla., the CDC dispatched 12 disease fighters to the West Palm Beach area to help state and local health officials track down the source of the malady and contain it.

A week later, as anthrax spores started turning up in letters in New York and Washington, the CDC shifted into high gear. An anthrax crisis operations center was quickly set up at the CDC's Atlanta complex. Its laboratories shifted into a 24-hour, seven-days-a-week schedule to analyze thousands of samples coming in from the field for anthrax spores.

Scores of workers were pulled in from other CDC programs. Dozens of other CDC doctors, epidemiologists and other experts were dispatched to New York, New Jersey and Washington to assist state and local health officials and consult with the FBI on the crisis.

The 55-year-old CDC, the nation's premier disease-fighting agency, is no stranger to epidemics, but the anthrax outbreak is the agency's first real trial by fire, albeit a limited one, on the intentional release of a deadly germ -- a bioterrorist attack.

Facilities are lacking

Early in the outbreak, the CDC came under criticism. Health officials, emergency management authorities, the news media and others complained the agency was withholding information about the outbreak.

Health and Human Services Secretary Tommy Thompson himself said early on that Stevens' death in Florida was an "isolated case," and there was no apparent terrorist plot. A few days later, Thompson reversed himself -- it was indeed a terrorist plot.

The CDC said it was muzzled by the FBI because of the criminal investigation. That restraint has been lifted, and the CDC now is holding daily press briefings.

Thursday, Bush, in a visit to the CDC , praised the agency's employees for their "hard work," though he stopped short of proposing funding beyond what is included in a $40 billion anti-terrorism initiative.

The anthrax crisis has drawn national attention to the CDC's physical shortcomings. Much of the agency, whose workers are spread over the main complex near Emory and more than 20 additional locations around metro Atlanta, is seriously overcrowded.

The situation prompted U.S. Sen. Max Cleland (D-Ga.) to remark after a recent tour: "The CDC has first-class people to do the job, but they must work in second-rate facilities."

Cleland and U.S. Rep. Saxby Chambliss (R-Ga.) have promised to seek hundreds of millions of dollars more for the CDC beyond its current budget. In addition, Cleland, agitated over the FBI's earlier clampdown on the CDC, introduced a bill last week to make the CDC the chief information source in a national health emergency and to clarify the agency's role in relation to the FBI.

Two waves of cases

Despite the CDC's run-down facilities, agency officials say they have been able to deal adequately with the anthrax crisis. They take heart in the fact that the epidemic is relatively small so far.

A chart released last week showed that diagnosed anthrax cases fall into two clusters, suggesting that cases resulted from contaminated letters sent in two waves.

Still, says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, the fact that the outbreak is the result of bioterrorism makes the situation "unpredictable."

Contaminated mail has not been able to explain the source of one of the inhalation anthrax cases, that of Kathy Nguyen, a New York hospital worker who died Nov. 2. Her death raised disturbing thoughts as to how she became infected.

One possibility is that at some point she crossed paths with a bioterrorist spreading anthrax spores. If that is the case, reconstructing her movements might lead to the source.

Gerberding said last week "no stone is being left unturned" in searching for the source of Nguyen's infection, but she acknowledged that the investigation has turned up few clues.

Old theory challenged

An even more disturbing possibility, say some health experts, is that Nguyen came nowhere near the perpetrator who distributed the spores, and was infected anyway. She may have been infected by a very few spores that she somehow happened to breathe in.

If that is the case, it would back up what many anthrax authorities are beginning to suspect: that the old theory, which holds that a person has to breathe in at least 8,000 spores to become infected, no longer holds true.

No matter how the current anthrax crisis turns out, CDC officials say the outbreak already has taught them some valuable lessons that will help them deal more effectively with the next bioterrorism attack.

Still, the basic questions in the current outbreak remain unanswered. Where did the anthrax spores come from in the first place? Were they produced by a domestic bioterrorist group or a foreign group connected with Osama bin Laden or another terrorist?

Crude delivery method

This much can be deduced so far from the anthrax crisis: Someone or some group has a supply of the anthrax bacterium, and they have a basic know-how and at least some crude equipment to grow the germ. They also have basic expertise and equipment to mill anthrax spores into tiny particles so that they can be easily inhaled and sucked deep into the lungs.

What they apparently don't have is an effective system for spreading the spores widely so that they infect thousands. Disseminating the spores by envelopes in the mail is a crude delivery method, bioterrorism experts say.

"Right now, someone is firing a powerful bullet through an ineffective gun," says Dr. Michael Osterholm of the University of Minnesota.

Of greater concern is that bioterrorists may use even deadlier agents in the next attack. Anthrax, officials note, cannot be spread from one person to another, but even more virulent germs such as smallpox are highly contagious.

Law enforcement and public health officials emphasize they have no evidence that an attack involving smallpox is imminent. But they are aware that such an attack is a possibility, and the CDC has vaccinated about 140 of its workers against the disease so that they can move in quickly and stymie the spread of the malady should an outbreak occur.