Doctors don't know yet if the swine flu that may have killed at least 149 people in Mexico is more like the 1918 form of influenza that left 50 million people dead, or a 1976 version that was fatal to just one.

Either way, the lessons from past pandemics in 1918, 1958 and 1968 may help reduce the severity of the current threat, said Barry Bloom, a professor at Harvard School of Public Health in Boston. The most basic precautions, including hand washing, avoiding close contact with the sick and covering the nose and mouth during coughs and sneezes, remain the same, according to the World Health Organization.

A flu pandemic that's similar in scope to the 1918 pandemic could kill 71 million people worldwide and push the economy into a "major global recession" costing more than $3 trillion, according to a worst-case scenario published by the World Bank in October. An outbreak that follows the route of the 1976 event could end up doing little more than frightening people enough so they increase their precautions.

It's just too early to tell, Bloom said. "The answer to virtually everything is that no one knows enough yet," Bloom said in a telephone interview. "The flu is incredible in its unpredictability."

Higher Alert

The WHO raised its pandemic alert level yesterday to 4 from 3, acknowledging the growing threat of a global outbreak. Still, it's not inevitable that swine flu will continue to grow as a problem, said Keiji Fukuda, assistant director-general for health security and environment.

The U.S. government has already declared swine flu a public health emergency. Signs that the threat is growing to pandemic levels may jumpstart health planning based on assumptions from the 1918 influenza epidemic, according to the Department of Health and Human Services.

Some people will be infected and won't show symptoms, the assumptions suggest. That could help extend individual outbreaks to 6 to 8 weeks, with events that include multiple communities lasting two to three months, according to the HHS web site.

As many as 152 people have died in Mexico from flu-related causes, and 64 cases have been confirmed by the CDC. In New York, hundreds of staff, family and students at three schools were sickened, said Mayor Michael Bloomberg, who is also the founder and majority owner of Bloomberg News parent Bloomberg LP.

Two-Week Supply

Community plans should include the possibility that hospital services, banks, stores, restaurants and post offices won't function. As a result, the HHS suggests storing a two-week supply of water and food. Nonperishable items that may be useful in a pandemic include canned meats and vegetables, dry cereal, canned juice, crackers, and granola, the HHS says.

The first precautionary steps by the U.S. government have already been taken. The Strategic National Stockpile will release a quarter of its antiviral drugs to help respond to the outbreak, according to information on the CDC Web site.

The most striking similarity between 1918 and the current flu is the age distribution of the people who died, Bloom said. The seasonal flu typically kills infants, the elderly, pregnant women, and the chronically ill. The 1918 pandemic, by contrast, killed a disproportionate number of people between the ages of 20 and 40, according to the CDC.

"In 1918, the mortalities occurred in healthy people in their 20s, which is what we're seeing in Mexico," said Bloom. "That's a bit of a worry."

Median Age

The median age of infection in the U.S. is 16, affecting people from ages 7 to 54 years, according to the Centers for Disease Control.

The 1918 flu was first reported in March of that year, in Kansas's Haskell County, according to the U.S. Department of Health and Human Services. The flu then became dormant in the U.S. for the summer, re-emerging in a more severe form in the fall of 1918, with a final wave of illness in the spring of 1919.

It's still too early to know whether the current disease is more like 1918 or 1976, when an outbreak of swine flu broke out in Fort Dix, sickening 13 soldiers and killing one, said James Thomas, an associate professor of epidemiology at the University of North Carolina's school of public health.

Because influenza is difficult to model, it's hard to predict how the virus will spread, Thomas said.

"In 1976, people acted before thinking things through fully," Thomas said. "This is not the time to overreact. It's still early, and we don't know how serious it will be. It could be mild."

'Animalistic Fear'

The current swine flu may remain relatively mild, since it's a relatively new virus, said Marc Siegel, an associate professor at New York University School of Medicine, and the author of False Alarm: the Truth About the Epidemic of Fear. Part of the problem in 1918 was that people didn't know what was going on and underreacted, Siegel said.

There's a different worry now, he said in a telephone interview. "I know we're going to overreact, Siegel said. "You can't approach this from animalistic fear. When you panic and are afraid, you don't take proper precautions."

The legacy of the 1918 pandemic is in precautions, said Stephen Morse, an epidemiologist at Columbia University's Mailman School of Health in New York. People who feel sick should stay home, and the very sick should see a doctor, Morse said. The virus is sensitive to antiviral drugs such as Roche Holding AG's Tamiflu and GlaxoSmithKline's Relenza, Morse said.

"Use the same hygienic measures your grandmother told you," Morse said in a telephone interview. "And remember to wash your hands."