Cancer patients at the University of Washington Medical Center and Seattle Cancer Care Alliance may have been exposed to tuberculosis by a health worker diagnosed last month with an active infection.

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About 140 cancer patients may have been exposed to tuberculosis by a health-care worker who was diagnosed with an active case of the disease, officials with the University of Washington Medical Center and the Seattle Cancer Care Alliance said Tuesday.

Patients who had contact with the worker between February and July are being advised to be tested for the infection, which can cause serious illness in people with weakened immune systems.

“We believe the risk of this person passing TB to others is low,” said Dr. Masa Narita, TB disease-control officer with Public Health – Seattle & King County.

Although tuberculosis is infectious, it doesn’t spread as easily as the flu, for instance, he added.

In addition to patients, 47 staff members who worked closely with the person have been tested, with no positive results based on preliminary and final results.

The health worker, who was not identified, was infected with tuberculosis five years ago while working with a high-risk patient at another hospital, said Dr. Steven Pergam, director of infection control at SCCA. The worker developed latent, or dormant, infection, and was treated with prophylactic drugs to prevent active infection.

However, it appears the treatment failed — a “very rare” occurrence, Pergam said.

Health workers are permitted to care for patients while infected with latent tuberculosis because the disease is dormant and not contagious, Pergam said. About 3 percent of health-care workers in the U.S. may have latent tuberculosis, according to a 2012 study of cases reported to the Centers for Disease Control and Prevention.

The health worker developed a cough, a symptom of tuberculosis, in mid-May, but doctors believed it was caused by another unidentified condition, Pergam said. When the cough didn’t respond to treatment, tuberculosis was suspected, and the worker was placed on leave in mid-July. Infection was confirmed in late July.

That left more than two months when the worker had active tuberculosis while working with cancer patients, experts speaking to reporters acknowledged.

“We absolutely understand this can be concerning news for our cancer patients,” said Dr. Tim Dellit, UW’s associate dean for clinical affairs.

Patients will be tested now and again in eight to 10 weeks. Cancer patients infected with tuberculosis may develop active disease quickly, within weeks or months instead of years, Pergam said.

Although the health worker’s colleagues tested negative for the disease, one close contact was positive for latent tuberculosis. The person spent time in a part of the world where tuberculosis is common and is not believed to have infected the health worker.

Officials declined to provide any information about the worker, including gender or age, citing privacy concerns. The worker is being treated with a common combination of drugs used for tuberculosis; there is no evidence that he or she has an infection resistant to treatment, Narita said.

A call line has been set up for patients, family members, caregivers and staff who have questions about the exposure. That number is 1-855-520-8600.

Tuberculosis is caused by bacteria that can be spread through coughing, sneezing, speaking or singing. Symptoms include cough, fever, night sweats and weight loss.

In 2015, 208 new cases were reported in Washington state, including 98 in King County.