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Two new studies of tuberculosis cases in New York City and San Francisco suggest that one in 10 TB sufferers is contagious and are fueling a recent resurgence of the potentially deadly lung disease.

Many doctors believe the majority of TB cases represent old infections that have become reactivated, often as immune systems begin to fail as a result of diseases like AIDS.But the new studies, published in Thursday's New England Journal of Medicine, suggest that belief is unfounded.

At least one third of the cases in the two cities studied were found to have been spread from person to person.

"We must now recognize that much of today's tuberculosis results from recent failures of treatment and public health measures," said Drs. Margaret Hamburg of the New York City Department of Health and Thomas Frieden of the Centers for Disease Control and Prevention in Atlanta.

In an editorial in the Journal, the doctors said the findings show how "a single patient can account for dozens of active cases, as well as hundreds of tuberculosis infections."

One infected person in San Francisco was found to have been responsible for one in 17 cases in the city.

Researchers who examined the cases in New York and San Francisco used chopped-up genetic material from the TB bacteria to produce a type of family tree among the various strains in the cities.

This allowed a team led by Dr. Peter Small of the Stanford Medical School to conclude that 191 of the 473 patients in the San Francisco area had only recently been infected. Those who did not take the medicine prescribed for the disease were the main source of the problem.

In the New York study, led by Dr. David Alland of the Montefiore Medical Center, the researchers concluded that recently transmitted TB was responsible for 40 percent of all the cases. In almost two thirds of the cases, the disease was found to be resistant to one or more drugs.