ASHINGTON, Sept. 29 — The United States is inadequately prepared to confront bioterrorist attacks, according to a broad range of health experts and officials. The nation must develop new vaccines and treatments, they say, but it must also fortify its fragile public health infrastructure, the first line of defense in detecting and containing biological threats.

Bioterrorism — the intentional release of potentially lethal viruses or bacteria into the air, food or water supply — poses daunting technical challenges, and experts say it would be difficult to carry out a successful attack. Still, many believe it is inevitable that someone will eventually try it in the United States.

In the weeks since the Sept. 11 attacks on the World Trade Center and the Pentagon, much of the discussion about bioterrorism has centered on a shortage of antibiotics and vaccines. But the bigger problem, officials agree, is a lack of basic public health infrastructure and preparedness that could thwart a terror attack or limit its effects.

Doctors are poorly trained to recognize symptoms of infection with possible biological weapons, like plague and anthrax, which can resemble the flu. Many of the nation's hospitals lack necessary equipment — in some cases even simple tools like fax machines — to receive or report information in an emergency. Though a number of federal agencies have established bioterrorism response teams and procedures, and there has been steady improvement in laboratory facilities around the country to test and identify biological agents, the result is a patchwork, set against a larger patchwork of cities, counties and states with their own reporting requirements and plans.

"For bioterrorism, the No. 1 inadequacy, if you had to rank them, is the inadequacy of our public health infrastructure," said Senator Bill Frist, Republican of Tennessee. "That is a product of about 15 years of neglect."

In a report issued last week, the General Accounting Office said the government's bioterrorism planning was so disjointed that the agencies involved could not even agree on which biological agents posed the biggest threat. Officials at the Centers for Disease Control and Prevention, for instance, consider smallpox a major risk. But the Federal Bureau of Investigation does not even put smallpox on its list.

At the same time, there are holes in the federal bureaucracy, where two important health positions remain unfilled: commissioner of food and drugs and director of the National Institutes of Health. The Food and Drug Administration will play a crucial role in the development of vaccines or treatments for use in the event of a biological attack, but President Bush and Congress — in particular Senator Edward M. Kennedy, Democrat of Massachusetts — have been unable to agree on an acceptable nominee.

Federal officials got a taste of how complicated, and chilling, a bioterrorist attack could be during a war game played at Andrews Air Force base, outside Washington, in June. The exercise, code-named Dark Winter, began with a report of a single case of smallpox in Oklahoma City. By the time it was over, the imaginary epidemic had spread to 25 states and killed several million people. As it unfolded, growing grimmer and grimmer, the government quickly ran out of vaccine, forcing officials to make life-and-death decisions about who would be protected — health workers? soldiers? — and whether the military would have to be brought in to quarantine patients.

"Dark Winter showed just how unprepared we are to deal with bioterrorism," said Jerome M. Hauer, the former head of emergency management in New York City and now a bioterrorism consultant to Tommy G. Thompson, the secretary of the Department of Health and Human Services. "It pointed out that there were significant challenges to all levels of government."

To meet those challenges, Senators Kennedy and Frist are urging President Bush to spend at least $1 billion on a range of measures that, they say, will improve the ability of health officials to combat bioterrorism. In an interview, Mr. Thompson agreed that improvements were needed, although he said the government was prepared to handle an attack right now.

"I would like to expand our pharmaceutical supplies," Mr. Thompson said. "I would like to strengthen the public health system. I would like to get some more inspectors for the food supply. I would like to expand security in our laboratories. I would like to purchase more vaccine."

For years, federal officials considered the threat of bioterrorism to be negligible. But concern began to mount in 1995, after a Japanese cult, Aum Shinrikyo, launched nerve gas attacks in the Tokyo subways. In the wake of the World Trade Center and Pentagon attacks, some members of the public have developed intense fears of germ warfare, and are trying to stock up on their own supplies.

"We have people buying gas masks and antibiotics when that is not going to provide real protection," said Stephen S. Morse, director of the Center for Public Health Preparedness at Columbia University.

Mr. Thompson said the administration was "very confident that we could act and react to any kind of bioterrorist breakout." But while Dr. Morse and other public health experts say the nation is better prepared than it was even three or four years ago, they do not share that confidence.

For instance, the United States has only 7 to 15 million doses of smallpox vaccine on hand — estimates vary — while experts estimate that at least 40 million would be needed to combat a serious epidemic. Under a government contract, a company in Cambridge, Mass., is testing a new vaccine, but it will not be available until 2004 at the earliest.

But perhaps the most pressing need, many health experts say, is improving the nation's ability to recognize when a biological attack is under way.

"We are not going to have a bomb fly out of the sky and land on somebody so that we can say, `Look, there's a bomb, and we are all dying of anthrax,' " said Asha M. George, who studies biological warfare for the Nunn-Turner Initiative, a nonprofit foundation in Washington. "It is most likely going to be a covert release, and people will get sick and go to their hospitals, and the public health system will have to pick up on this."

In some ways, the Sept. 11 attack on the World Trade Center was a test of that system.

Minutes after two jets slammed into the World Trade Center, the National Guard was mobilized. The Guard has created 29 teams around the nation to aid the response to chemical, biological and radiological attacks; on Sept. 11, a 22-member unit was ordered into Manhattan to test the air for deadly germs or chemical toxins. None were found.

Soon afterward, the Centers for Disease Control and Prevention, the branch of the health and human services agency that coordinates bioterrorism preparedness, alerted state and local health departments to look for signs of unusual illnesses that might be the result of a biological or chemical attack. That alert remains in effect; so far, nothing out of the ordinary has been reported.

At the same time, officers from the centers' Epidemic Intelligence Service were stationed at 15 sentinel, or warning, hospitals scattered in New York City's five boroughs, also looking for strange symptoms. And for the first time, drugs and other medical supplies were dispatched from the National Pharmaceutical Stockpile, which is maintained by the disease control centers to respond to a germ outbreak.

"You could see how orderly we are right now in responding to the terrorist attack on the 11th," Mr. Thompson said. "And we would do the same thing with a bioterrorist attack."

But in many respects, Sept. 11 was not a true test. There were no biological or chemical agents to detect. Because there were far fewer people injured than officials had originally expected, the epidemic intelligence officers were working in relatively calm hospital surroundings, as opposed to crowded emergency rooms. The drugs and medical supplies went largely unused.

So while Mr. Thompson insists the government "can handle any contingency right now," there is no way to know if the response would have been adequate during an actual bioterrorism attack, according to one expert closely involved in the government's antiterror planning who spoke on condition of anonymity.

For one thing, the expert said, in the New York City attacks doctors, nurses and other health care workers stayed at their jobs. But in the event of a biological attack, many might go home to their own families.

Moreover, with managed care's pressure to eliminate hospital beds and increase efficiency, hospitals have lost their so-called surge capacity — the ability to accommodate a sudden increase of patients. And doctors are not trained to recognize the symptoms of germ warfare.

"When you don't see very uncommon things, you don't think about very uncommon things," said Nicole Lurie, a former federal health official who worked on bioterrorism issues in the Clinton administration. "I saw three people in the morning yesterday with acute respiratory illness. They all had the same symptoms. Should I think this is bioterrorism?"

A big part of the government's formidable challenge is simply coordinating its response; across Washington, a range of bureaucracies, including the departments of energy, defense and justice and the health and human services agency, are busy planning for bioterrorist attacks. That job will soon fall to Tom Ridge, the governor of Pennsylvania, whom President Bush named to head a new Office of Homeland Security.

Some experts outside government say Secretary Thompson has already taken a step in the right direction by creating a position coordinating a departmentwide initiative against bioterrorism. In July, nearly two months before the World Trade Center attacks, Mr. Thompson named Scott Lillibridge, the disease control center's top expert in bioterrorism, to fill the job.

So, despite their worries, many experts agree that the groundwork has been laid for improvements.

"Are we prepared to prevent it? No," Dr. Lurie said. "Are we prepared to respond to it? It depends on what form it takes. I would say that we are a whole lot further along than we were three or four years ago."

Mr. Hauer agreed. "A lot of what we need to do is being done," he said. "The problem is, some of these steps take time."