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.