International health experts say that two of the three conditions for an avian flu pandemic in Southeast Asia have already been met. First, a new strain of the virus, called A(H5N1), has emerged, and humans have little or no immunity to it. Second, this strain can jump between species. The only remaining obstacle is that A(H5N1) has not yet mutated into a form that is easily transmitted from human to human.

However, there have been some alarming developments. In recent months, the virus has been detected in mammals that have never previously been infected, including tigers, leopards and domestic cats. This spread suggests that the virus is mutating and could eventually emerge in a form that is readily transmittable among humans, leading to a full-blown pandemic. In fact, according to government officials, a few cases of human-to-human spread of A(H5N1) have already occurred.

The precedent that experts fear is the 1918 flu pandemic, which began in the American Midwest and swept the planet in the era before air travel, killing 20 million to 40 million people. As John M. Barry, author of "The Great Influenza," has observed, "Influenza killed more people in a year than the Black Death of the Middle Ages killed in a century; it killed more people in 24 weeks than AIDS has killed in 24 years."

At the moment, effective responses to an avian flu pandemic are limited and will come far too late for many people in Southeast Asia. Indeed, so far more than 60 percent of those diagnosed with the avian flu have died. There is no proven vaccine for the A(H5N1) strain, and it could take months to produce a fully effective one. Moreover, while some antiviral treatments may help flu sufferers, they are not widely available and must be administered to patients within 24 hours after the onset of symptoms.

It is essential for the international community, led by the United States, to take decisive action to prevent a pandemic.

So what should we do? Recently, the World Health Organization called for more money and attention to be devoted to effective preventive action, appealing for $100 million.

Congress responded promptly. A bipartisan group of senators obtained $25 million for prevention efforts (a quarter of the request, the traditional contribution of the United States), allowing the CDC, the Agency for International Development, the Health and Human Services Department and other agencies to improve their ability to act.

In addition, the Senate Foreign Relations Committee unanimously approved legislation directing President Bush to form a senior-level task force to put in place an international strategy to deal with the avian flu and coordinate policy among our government agencies. We urge the Bush administration to form this task force immediately, without waiting for legislation to be passed.

But these are only modest first steps. International health experts believe that Southeast Asia will be an epicenter of influenza for decades. We recommend that this administration work with Congress, public health officials, the pharmaceutical industry, foreign governments and international organizations to create a permanent framework for curtailing the spread of future infectious diseases.

Among the parts of that framework could be these:

• Increasing international disease surveillance, response capacity and public education and coordination, especially in Southeast Asia.

• Stockpiling enough antiviral doses to cover high-risk populations and essential workers.

• Ensuring that, here at home, Health and Human Services and state governments put in place plans that address issues of surveillance, medical care, drug and vaccine distribution, communication, protection of the work- force and maintenance of core public functions in case of a pandemic.

• Accelerating research into avian flu vaccines and antiviral drugs.

• Establishing incentives to encourage nations to report flu outbreaks quickly and fully.

So far, A(H5N1) has not been found in the United States. But in an age when you can board planes in Bangkok or Hong Kong and arrive in Chicago, Indianapolis or New York in hours, we must face the reality that these exotic killer diseases are not isolated health problems half a world away, but direct and immediate threats to security and prosperity here at home.