A passenger on an American Airlines flight from India who sat near a woman with drug-resistant tuberculosis has tested positive for TB, federal health officials said.

The Centers for Disease Control and Prevention uncovered the second case after tracking down and testing more than two dozen passengers who sat near the woman on the December flight from New Delhi to Chicago.

But CDC officials cautioned Thursday that because the other passenger had lived in a country where TB is common, they could not determine how the infection was contracted.

"It's not really surprising to see people on a plane from a country with high TB prevalence testing positive for TB," said Shelly Diaz, a spokeswoman for the agency.

The 30-year-old woman diagnosed with the full-blown version of the disease late last year arrived in Chicago on Dec. 13 and then traveled on to San Francisco International Airport. Health officials said the woman learned in India that she was sick with TB but still boarded the international flight.

About a week after the flight landed, the woman showed up at the Stanford Hospital emergency room with a fever and chest pain and coughing up blood.

The Silicon Valley woman has since been released from the hospital after months in isolation but will remain quarantined at home for several more weeks, local health officials said.

"We don't really want anyone right now to come into contact with her," said Dr. Marty Fenstersheib, health officer for Santa Clara County.

The woman continues to take antibiotics, and a health care worker directly observes her taking her medication every day, a standard protocol for TB, Fenstersheib said. She may have to take drugs for up to two more years to completely eradicate the disease, he said.

To determine whether the woman was the one who infected the other passenger, that person would have to develop the disease rather than just test positive for the germs that cause it, Diaz said.

Otherwise doctors have no way to test the TB bacteria to see if the strains match.

People who carry the germ that causes TB but do not have the active version of the disease are not contagious. Many people who carry TB do not develop symptoms that allow the disease to spread.

"This person is not a health threat at this time," Diaz said.

Under World Health Organization guidelines, a total of 44 passengers sitting within two rows of the infected woman on the flight from India to Chicago were considered at risk.

The CDC oversaw testing for 27 passengers. Others reported already being TB carriers, while some returned to their home countries before they could be tested.

Passengers who only sat near the woman on the Chicago-to-San Francisco flight were not exposed long enough to be infected, according to the CDC.

Last year, a TB patient caused an international health scare when he flew to Europe for his wedding, though no evidence has emerged that he spread the disease.