New cases of tuberculosis in Chicago have dropped to an all-time low, according to a study released by the Chicago Department of Public Health, with blacks seeing the biggest decline in newly-diagnosed cases in 2008.

At the same time, more than half of new cases are coming from the city’s immigrant community, suggesting many people bring the disease when they arrive in the United States.

TB cases in immigrants rose to 54 percent of all cases in 2008, from 51 percent in 2007. About 45 percent of new cases in 2006 originated from foreign-born residents. That statistic has been rising since 1993, according to public health department figures.

“This is the second year that more than half of all newly-diagnosed cases are among foreign-born Chicagoans,” says CDPH spokesman Tim Hadac. “It is a trend that has been going on for some time - although it should certainly not be seen as a knock against recent immigrants - because some of the cases are doubtless among foreign-born who have been here for years."

Dr. John Flaherty, associate chief of infectious disease at Northwestern University's Feinberg School of Medicine, says one-third of the world's population has TB.

"The way TB works is you get infected with it typically, and it sort of lays dormant for a time. Then over a period of months or years, it can reactivate," he says. Often stress, aging or other diseases cause TB to flare up.

Overall, newly-diagnosed cases dropped from 258 in 2007 to 214 in 2008 -- the lowest ever. The CDPH study found the biggest decline in new cases was among blacks -- there were 124 new cases diagnosed in 2007, and 86 in 2008.

University of Chicago pulmonologist and professor of medicine Alan Leff says the number of foreign-born cases in Chicago mirrors national trends, but is encouraged by the decreasing overall numbers.

“There once was a time when we were seeing approximately 1,200 cases a year. It’s a step in the right direction, for sure,” he says.

Leff says he is most concerned with those infected with HIV and drug addicts, who might not have access to treatment.

Officials attribute the decrease in cases in part to the city’s Directly Observed Therapy (DOT) program, in which public health workers observe patients swallowing their TB drugs. Before DOT became widespread in the 1990s, nearly half of Chicago’s TB population failed to complete a six-month treatment regimen. Today, more than 90 percent complete the treatment within a year.

Terry Mason, Chicago's public health commissioner, says:  “The challenge for us now is two-pronged: To maintain local support for TB elimination efforts, and to insist that national and international organizations devote comparable levels of resources for TB control efforts around the world.”

“This is all good news, and I think the pot of gold at the end of the rainbow, it might actually be possible to eradicate TB,” Leff says.

TB kills more than two million people worldwide annually. Flaherty, of Northwestern, says new FDA-approved blood tests are available for patients who don't want to want a skin test.