(The New York Times News Service) -- The rapid spread of the influenza virus called swine flu to the United States and other nations has changed what was a serious epidemic confined to Mexico into a potential "pandemic" -- a disease that threatens to spread around the world before it is stopped.

However, the emerging health crisis, which recalls a long history of similar epidemics that have turned into major global threats, may not get that far. Experts in communicable diseases say there are now many lines of worldwide defense that could quickly end this new swine flu threat.

"We have a lot more tools to combat a pandemic of flu viruses today than we have ever had before," said Dr. Tomas Aragon, who heads the UC Berkeley center for infectious diseases and emergency readiness.

Besides the relatively new influenza anti-viral drug called Tamiflu, which is known to be effective against this version of swine flu, Aragon noted, scientists are now able to determine the specific genetic subtype of a flu virus involved in an outbreak, which helps them develop highly specific vaccines.

There are also many new antiviral drugs available to fight the virus, as well as antibiotics to combat secondary infections.

Tamiflu and another anti-viral drug called Relenza are now stockpiled by the government and are being released as needed, Aragon said.

By raising the pandemic alert level on swine flu from phase 3 to 4 Monday, the World Health Organization confirmed that the disease was now spreading through human-to-human contact and that community-level outbreaks outside of Mexico had been found.

The earlier Phase 3 alert, according to Aragon, meant that the new swine flu virus subtype in Mexico had infected humans there, but there was as yet no evidence the disease could be spread by human-to-human contact nor that it had spread to other countries beyond its origin.

"Phase 4 indicates a significant risk of a pandemic but does not necessarily mean that a pandemic is a foregone conclusion," according to the WHO definition.

Two higher, more alarming pandemic alert phases are possible but so far seem unlikely, WHO officials said Monday. Phase 5 would mean that human-to-human transmission of the virus is spreading to more than one region of the world, and that "a pandemic is imminent," and a phase 6 alert "indicates that a global pandemic is underway," according to WHO's definition.

Major epidemics have a long history, and the infamous "Spanish Influenza" of 1917-18 is a classic example. It broke out during World War I among American soldiers in Kansas, spread swiftly around the world, and before it ended 18 months later, had killed more than 50 million people, including an estimated 675,000 in the United States.

Viruses were virtually unknown at that time, and the cause of the Spanish Flu was also a mystery, but scientists later analyzed tissue samples found in Alaska permafrost and reconstructed the virus, calling its subtype H1N1.

In the summer of 1957, the Asian Flu pandemic spread from China to the United States and quickly spread among schoolchildren in the fall, even though a vaccine against it had recently been introduced. Although it was much milder than the pandemic of 1917-18, its global death toll was estimated at more than 2 million.

The most recent influenza pandemic became known as the Hong Kong Flu of 1968, and it caused 34,000 deaths in the United States -- mostly among the elderly -- and about the same number in other countries.

A frightening outbreak of what appeared to be influenza broke out in January 1976 among soldiers at Fort Dix, N.J. Scientists at the Federal Centers for Disease Control identified it as a new strain of swine flu, and fears of a major new pandemic led President Gerald Ford, together with officials of the Center for Disease Control and Prevention and the National Institutes of Health to declare an emergency and begin mass immunization with a specific vaccine.

Within 11 months, 40 million civilians were vaccinated, but reports that a rare neurologic disease called Guillain-Barre syndrome had developed after some vaccinations led the CDC to recommend halting the campaign, and the vaccination campaign ended on December 16, 1976. The syndrome's connection to the flu vaccine was never proven.

Copyright 2009 The New York Times News Service. All rights reserved.