Global health officials are worried about the spread of a new flu that has killed some 150 people in recent weeks and has the potential to create a pandemic. This alarm confirms warnings that have been issued since the SARS (severe acute respiratory syndrome) outbreak of 2003 — with two important differences: This disease seems to have originated in Mexico, and is not a variant of the avian flu that has been the focus of epidemiologists for the last several years. This new influenza virus, thought to be a form of swine flu, is a reminder of the need to remain vigilant and to expect surprises in the fight against deadly disease.

The last major flu pandemic was in 1968, when the Hong Kong flu killed about 1 million people worldwide. That was a relatively mild outbreak; the Spanish flu pandemic that followed the end of World War I claimed between 20-100 million lives. Pandemics strike with alarming regularity. If, as experts believe, influenza pandemics occur every 28 years on average, then we are overdue. Some thought SARS might have been the next big outbreak. SARS was alarming: the disease was first noticed in November 2002 and quickly spread from southern China to infect more than 8,000 people in some 37 countries around the world. But that does not measure up to truly global pandemics. It was not extremely virulent — it caused 774 deaths, a fatality rate of “just” 9.6 percent.

The SARS outbreak focused the attention of public health specialists on Asia and on avian flu, which has appeared with worrisome regularity. It has been found among bird species worldwide, and hundreds of millions of birds have been killed to eliminate reservoirs where the virus can generate. More than 260 deaths have occurred, but the virus remains relatively stable and has not mutated to into a form can be transmitted person to person. This has prevented it from becoming a real threat.

With most eyes elsewhere, there has been an outbreak of swine flu in Mexico. Public health officials there report some 2,000 suspected or confirmed cases, with some 150 deaths. The outbreak has occurred throughout the country, although there has been a concentration of cases in Mexico City, home to some 20 million people. The crowded living conditions there facilitate transmission of disease. The Mexican government has canceled large public events and closed museums and schools to help stop the spread. These actions may have been too late, though. The disease has already been detected in several states in the United States, although there have been no deaths there yet.

During the SARS outbreak, there were fears that governments were hiding cases of the disease. That does not appear to be the case this time. Mexican health authorities observed an upsurge in flu cases in recent weeks, but they assumed it was a late rebound of the flu season, a periodic occurrence. When U.S. officials announced they had found a new flu strain, the Mexican authorities tested their samples and cases against the U.S. results and discovered their virus was the same one. No matter what the cause, however, the result is the same: Opportunities to contain the disease have been lost. Suspected or confirmed cases cases have also been reported in 13 other countries including Canada, Britain, France, Spain, South Korea, Australia and New Zealand.

Though called a “swine” flu, the new virus is actually composed of genetic segments of swine, avian and human flu viruses. That combination could explain a troubling element of this outbreak: Many of its victims are between the ages of 20 and 40. Most flu kills the weakest members of society, infants and the aged; this group of victims, however, is usually strong enough to resist the disease and it is also the victim profile of a pandemic.

The World Health Organization has raised its alert level for a global influenza pandemic threat from the earlier phase 3 to phase 4 — two notches below the level for a full-scale pandemic — since epidemiological data demonstrated human-to-human transmission, which can cause “community-level outbreaks.” WHO pointed to an increased likelihood of a pandemic but said that a pandemic is not “inevitable.”

Caution is wise. Premature declarations could cause panic. Instead, rigorous surveillance of flu cases is needed by all health services. Travelers need to be checked and isolated if they show symptoms, but bans are excessive at this point. Coordination among those authorities is especially important given the speed with which disease can spread. Public health facilities should be prepared to handle large numbers of infected patients. Thus far, the disease has been responding well to treatment by anti-viral medications, but human beings do not have immunity to animal-born viruses, which means that vaccines have to be developed. Those facilities should also be bolstered, but that too takes time.

This outbreak is a reminder of the need to be prepared for surprises. Health officials have been expecting a flu outbreak, but they anticipated it would start in Asia. Nature rarely conforms to our expectations. That is no excuse for not being ready.

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