As health officials announced nine probable cases of swine flu in Illinois, including five in Chicago, experts say the city is uniquely positioned to suffer an outbreak of the virus.

A large, mobile Mexican population, dense quarters and tight public transportation increase the risk, says Dr. Paula Kagan, an associate professor of nursing at DePaul University.

"Certainly, every location has their own risk potentials," Kagan says. 'If you compare us to LA or New York or any large port of entry...we're at risk because we're a large population."

"But," she says, "we also have a very good public health system."

Dr. Ronald Hershow, an associate professor of infectious diseases at the University of Illinois at Chicago, says the city's large Mexican population could contribute to the spread of the virus, which originated in Mexico. About 18 percent of Chicago's population is Mexican, according to 2000 Census figures.

"Given that the epicenter of this evolving situation is in Mexico and we know that there's a lot of travel and interchange between Mexico and Chicago, I think we can anticipate that more cases will arise here, and we have to keep track of it," he says.

City officials yesterday urged the public not to single out Chicago's Hispanic population as the city grapples with swine flu.

Michael Bolland, director of Chicago's aviation department, said yesterday that signs were placed across O'Hare and Midway airports in an effort to educate people about swine flu symptoms. Raymond Orozco, the city's emergency management chief, said similar measures were taken at CTA stations.

But David Ehrlich, a visiting assistant professor at DePaul's school of public service, says education is just the first step officials should take with regards to public transit.

"They should be doing everything they can to reduce the number of people and the amount of crowding, including encouraging people to work off-hours so people aren't crowded during rush hour," says Ehrlich, who studies mass transit systems. "Just doing that could substantially reduce crowding."

City officials have not told people to avoid public transportation or air travel, and continue to encourage people not to panic, as it evaluates the swine flu threat.

Still, Ehrlich says, there may come a time when Chicago's oft-overcrowded, under-funded public transit system will have to confront swine flu, including adding more buses and trains to certain routes to reduce the number of passengers.