he terrorist attacks on the World Trade Center and the Pentagon have ratcheted up fears that an even more terrible assault could lie ahead — this time with biological or chemical agents. Nothing that happened on Sept. 11 has changed the underlying reality that biological and chemical weapons are extremely difficult to make and disperse. But the attacks have changed our perception of the malevolence and determination of today's terrorists. The suicidal zealotry of the men who flew airliners into buildings, their willingness to prepare for years, their desire for mass casualties and their deep-seated hatred of Americans leave little doubt that they would escalate to even more dreadful weapons if they could.

That said, it is important to be realistic about the threat and how best to combat it. Despite loose talk that a college chemistry major could whip up a lethal weapon in his garage, the historical record suggests that it is nowhere near that easy. Selecting and growing a potent biological agent, maintaining its virulence, making the germs hardy enough and stable enough for widespread dissemination and finding an efficient means of spreading them around all pose obstacles that have frustrated even well-financed state weapons programs.

The Aum Shinrikyo cult in Japan, with more than $1 billion in assets, an apocalyptic theology that sanctioned mass killing and a corps of scientists and laboratories dedicated to producing lethal agents, launched two nerve gas attacks that killed 19 people, but failed repeatedly — as many as a dozen times over five years — to spread lethal biological agents. That does not mean that a group like Osama bin Laden's network, which also has ample resources and educated followers, would necessarily fail. But it would face enormous difficulties. The danger could escalate, of course, if disaffected Russian scientists sold their expertise, materials and equipment to terrorists or if Iraq, which has its own chemical and biological weapons programs, lent the terrorists a hand.

Although opinions differ widely on whether biological or chemical attacks are likely in the foreseeable future, virtually all experts agree that the United States needs to strengthen its defenses. The Bush administration is already trying to root out the terrorist cells before they strike. Efforts must also be intensified to ensure that impoverished Russian scientists do not sell their secrets or their weaponry to the highest bidder.

There have been spates of rumors about the possible poisoning of reservoirs, raising the specter of vast populations felled by their drinking water. But most experts say reservoirs are an unlikely target. Chlorine that protects the water would neutralize most biological agents, and the vast quantity of water in reservoirs would so dilute any chemical or biological agent that it would probably be rendered harmless. A more worrisome threat, some say, would be the release of lethal agents into the air or water intakes of an office building, or in confined and crowded areas like railroad terminals or subways. Defenses against such possibilities — including better air filtration systems — will have to be one element of homeland defense.

Crop dusters have been grounded twice since Sept. 11 for fear they might be used to spread lethal agents, but many experts are skeptical of the threat. They say the planes would need to be refitted with specialized nozzles to disperse biological agents and would have difficulty carrying nerve gas without leaks that would kill the pilot or those who load the plane. The federal government is right to err on the side of caution, but the nature of the crop-duster threat needs to be better defined.

The first line of defense against a biological attack would not be heroic fire and emergency workers like those who raced to the World Trade Center but rather the public health system that detects and monitors disease trends and would be likely to encounter the dying victims first. New York and other major cities have made great strides toward preparedness in recent years, but nationwide there are too few medical personnel trained to spot biological attacks, a shortage of sophisticated laboratories to identify the agents, inadequate supplies of drugs and vaccines to counteract the threat, and few plans for setting up quarantines and emergency facilities to handle mass casualties. That could prove disastrous if terrorists introduced a highly infectious disease like smallpox.

The nation has shortsightedly allowed its public health agencies and facilities to weaken in recent years. Gearing them up to cope with a malevolent biological attack would have the added advantage of enhancing our ability to deal with infectious diseases whose causes are wholly natural. The investment would pay health dividends even if no biological or chemical attack ever materializes.