Debbie Ottaviano, a state Department of Health scientist, conducts a control test in the search for swine flu.

The Providence Journal / Kris Craig

Are we on the verge of another global flu pandemic, like the one that killed some 30 million to 50 million people in 1918?

Or will it be less deadly, like the forgotten pandemics of 1957 and 1968?

Or will the whole thing fizzle, like the SARS outbreak of 2003?

Experts say they don’t know. But health officials could have the answer within a few days, as more information emerges about the behavior of a virus that led federal officials to declare a public health emergency and the World Health Organization to raise the pandemic-flu alert level.

“The early footprints of a pandemic are clearly there,” says Dr. Stefan Gravenstein, a Providence geriatrician, who has researched influenza and worked on pandemic planning. A pandemic is a disease outbreak that spreads over a wide geographic area and affects a large proportion of the population.

The swine flu virus that originated in Mexico is raising alarms because it contains components of animal viruses that are unfamiliar to the human immune system –– influenza that usually infects pigs and birds. Unlike most swine and avian flu viruses, this one can infect people. Because it’s a novel strain, human defenses against it are weak. And worst of all, the virus can be transmitted from one person to another.

But what’s not known is how fast it travels from person to person, nor how many people one sick person typically infects. So far, most of the 68 U.S. victims have been people who traveled to Mexico or had contact with someone who did. How quickly and how powerfully will swine flu move beyond that sphere?

Another big question is what percentage of its victims this virus will kill. So far, swine flu has been mild in the United States, with only five hospitalizations and no deaths. In Mexico, some 152 people have died –– but it’s not clear how many Mexicans have been infected but didn’t die. “If we knew exactly how many people were infected in Mexico, we’d have a much better idea how to look at those deaths,” said Dr. John B. Murphy, chief medical officer of Rhode Island Hospital, which is in the early stages of preparing for a flu pandemic.

Such pandemics have happened, he said, every 20 to 30 years throughout history; archeologists have even found evidence of flu pandemics dating to the 12th century. “A typical pandemic will affect somewhere in the range of 40 percent of the population,” he said, while a typical seasonal flu epidemic affects 10 percent of the people who haven’t been immunized.

The virus’s makeup is only one determinant of how it will behave, says Gravenstein. The outside world matters, too. When people are kept close together –– such as soldiers in barracks –– the disease will spread more quickly. World War I was a major contributor to the 1918 pandemic.

Another factor is individuals’ immune systems, Gravenstein said. The swine flu contains a mixture of foreign and familiar components. The more times you’ve had the flu or received a flu shot, the more likely your immune system carries an antibody able to fight off the new strain. “We don’t know how much protection we have from that,” Gravenstein said.

Then there’s the weather. The flu spreads more readily in cold, dry air, apparently because the droplets from coughs and sneezes become lighter and stay airborne longer. Cold seasons are also when people are more likely to huddle together. Just sitting four feet away from someone instead of side by side makes a significant difference in one’s likelihood of getting infected, Gravenstein said.

The arrival of warm, humid summertime weather could quiet this swine flu, at least for a time. The 1918 pandemic started with a small springtime outbreak before exploding the following autumn.

Today, governments and communities will be preparing for that possibility. Indeed, a lull between spring and fall might be just enough time to develop a vaccine.

That’s one of the advantages that didn’t exist even a few years ago, says Dr. Leonard A. Mermel, medical director of the Department of Epidemiology and Infection Control at Rhode Island Hospital. Recently developed techniques are enabling faster development of vaccines.

More than ever before, Mermel says, people know how to treat and slow the spread of influenza. Global health officials have new skills and resources for tracking and containing infectious disease. Studies have shown that closing schools, urging people to avoid large gatherings, keeping the sick at home, and that oft-repeated advice to wash hands and cough into your elbow can reduce the number of people who fall ill, he said.

Dr. Mark A. Gendreau, an emergency-medicine doctor at the Lahey Clinic in Burlington, Mass., envisions three possible scenarios for what can happen next. In one, the virus continues spreading rapidly now and develops into a global pandemic. In another scenario, the virus “putters out and we never see it again.” In the third, the virus quiets down in the next week or two and reemerges in the fall.

Gendreau considers the last two scenarios the most likely. And if the last one plays out, that would allow time to develop a vaccine and ramp up production of Tamiflu, which can reduce the severity and duration of this swine-flu infection. Tamiflu is already in ample supply, he noted, because this year’s seasonal flu was resistant to it, so the drug wasn’t used.

“We have the tools to deal with this,” Gendreau says.

Meanwhile, the state Health Department is continuing to watch for the arrival of swine flu in Rhode Island, something Health Director David R. Gifford considers just about inevitable.

Health-care providers throughout the state have been calling the Health Department whenever a patient who has visited an affected area reports flu-like symptoms. In a few of those cases — about a half-dozen each on Monday and Tuesday –– health officials ask for a swab from the patient’s nose to be tested for influenza.

The state laboratories can identify whether the person has type A influenza, and whether that virus is one of three human subtypes. If they find type A flu, but cannot identify the subtype, then the sample will go to the CDC laboratory in Atlanta to be tested for swine flu. So far, the state labs ruled out swine flu in all the samples and none has been sent to Atlanta.

ffreyer@projo.com