WASHINGTON (Reuters) - Nearly two million dead. Schools and public transit closed for days or even weeks. Hospitals overwhelmed. This frightening picture of an influenza pandemic is envisaged in the official U.S. plan released this week. H5N1 avian influenza has killed 62 people and infected at least 122 since 2003 -- hardly an alarming number. But the virus is sweeping through poultry flocks and has moved into birds in Europe. Agriculture and health officials agree the H5N1 virus is probably unstoppable, and say it could mutate at any time to become a disease that sweeps just as easily among humans. Experts have warned for years that an influenza pandemic is overdue and have a good idea of what it would look like. Three influenza pandemics occurred in the last century. The 1918 pandemic was the worst, killing anywhere between 20 million and 100 million people globally, depending on the estimate. The 1957 pandemic killed an estimated 2 million people globally and the last one, in 1968, killed 1 million. "An influenza pandemic has the potential to cause more death and illness than any other public health threat," the Health and Human Services department says in its new flu plan, posted on the Internet at http://pandemicflu.gov. "If a pandemic influenza virus with similar virulence to the 1918 strain emerged today, in the absence of intervention, it is estimated that 1.9 million Americans could die and almost 10 million could be hospitalized over the course of the pandemic, which may evolve over a year or more." Why paint such a frightening scenario? "We felt that it would be important that we have a worst-case scenario to make sure our planning efforts were measured against that," said Dr. Bruce Gellin, a vaccine expert who is heading up HHS's pandemic influenza planning. WORST CASE SCENARIO "We don't know what the H5N1 virus will do. We don't know what any virus will do. But we felt that we would be best suited to have our preparations based on that worst case which is, in modern memory, in 1918," Gellin told reporters this week. And help from the government would be limited. A vaccine against H5N1 is not ready, and there are not enough drugs in stock yet to treat more than a fraction of victims. "It is unlikely that there will be sufficient personnel, equipment, and supplies to respond adequately to multiple areas of the country for a sustained period of time," the HHS plan cautions. It says schools may close for 10 days at a stretch or even longer. Public transport and office buildings may be closed to curb the spread of the virus. People should prepare for the emotional impact of "mass casualties and deaths among children" and well as "economic collapse or acute shortages of food, water, electricity, or other essential services," it says. People could not plan on turning to hospitals for shelter. The American Hospital Association has said its members already have limited beds and equipment and are short of nurses. Experts say despite medical advances the world is more vulnerable to flu than in 1918, with a higher, more urban population, far higher international travel and more sick and elderly people. Creating quarantines and closing borders would have limited effect as influenza can be transmitted a day before a patient starts showing symptoms. The HHS report says globally and nationally, a pandemic might last for more than a year, while disease outbreaks in local communities may last five to 10 weeks.