SCENARIOS: What the new swine flu might do

WASHINGTON (Reuters) - The new H1N1 swine flu has spread to nine countries, may have infected more than 2,500 people and killed dozens in Mexico, including a toddler from Mexico visiting Texas who became the first U.S. death.

The World Health Organization says it cannot be stopped and the best the world can do now is try to mitigate its effects.

Although it is not yet a pandemic -- a global epidemic of a new and serious disease -- it could quickly start one. Here are a few ways the situation could develop:

SEVERE PANDEMIC

This is the worst-case scenario. Every 30-40 years the world suffers an influenza pandemic, with a new strain of flu spreading rapidly, causing serious disease and killing hundreds of thousands of people within a few weeks.

The 1918 pandemic is considered the worst-case scenario. At least 40 million people died in 18 months and the epidemics passed over communities in waves.

But that was in the pre-antibiotic era, in a time when even simple infections killed people, no mechanical ventilators were available and vaccines were primitive at best. The general population had little idea of how diseases were transmitted.

Even so, experts predict a 1918-like flu would keep 40 percent of the workforce out at any time, with people either sick, caring for sick relatives or children out of school, or simply lying low. That would lead to shortages of supplies and even power outages.

If this kind of pandemic struck today, millions could die, world trade would slow to a crawl and many economies would crumble.

MILD PANDEMIC

This last happened in 1968. The H3N2 strain killed an estimated 1 million people. Experts predict that a strain of such virulence today would have less severe effects, with antivirals available that were not on the market 40 years ago.

Even with vaccines, drugs and better public education, ordinary seasonal flu kills between 250,000 and 500,000 people every year. The difference with a pandemic strain is that there would not immediately be a vaccine against it and it would likely cause serious disease among broader age groups than the very young, very old and immune-compromised who are generally vulnerable to influenza.

There could be disruptions to trade and travel, currency fluctuations, interruptions of supplies needed for manufacturing and a shortage of antivirals and the antibiotics needed to treat the "co-infections" that often accompany flu. Some reports predict a shortage of ventilators, and hospitals in many countries, especially the United States, already operate at full capacity and would be overwhelmed by an influx of newly sick people.

NO PANDEMIC

Everyone is hoping this flu strain will just fizzle out. Influenza is a promiscuous virus, readily swapping genes with other flu viruses in a person's or animal's body. It also mutates constantly. Both these factors mean it can quickly grow worse, or become milder. It could at any time lose its ability to transmit easily from one person to another, or could become as mild as regular, seasonal flu.

But it would take months to know whether this has happened. Flu strains often disappear over the summer months, re-emerging in late summer or early autumn. The 1918 flu strain did this with a vengeance.

For now, the World Health Organization says only that a pandemic is possible.

"It's probably premature to think of this as a mild pandemic, as a severe pandemic," the WHO's Dr. Keiji Fukuda told a briefing on Wednesday. "It is very clear that we cannot predict what the course of this will be."

(For a graphic on the flu's spread, click here#swine)

(Reporting by Maggie Fox; Editing by Xavier Briand)

(For more Reuters swine flu coverage, please click here: here)