WASHINGTON (Reuters) - The hardest-to-treat form of tuberculosis kills half the people who get it, according to a South Korean study that is one of the few to track survival rates from the condition called extensively drug-resistant TB.

Tuberculosis is an infectious bacterial disease typically attacking the lungs. Increasing numbers of cases of TB that defy standard medical treatment are appearing worldwide.

The study tracked 1,407 patients with two categories of TB: multidrug resistant TB, or MDR-TB, which resists at least one of the two main TB drugs, and extensively drug-resistant TB, or XDR-TB, which defies nearly all drugs used to treat TB.

Forty-nine percent of those with XDR-TB died compared to 19 percent of patients with ordinary MDR-TB, researchers led by Dr. Tae Sun Shim of Asan Medical Center in Seoul wrote on Thursday in the American Journal of Respiratory and Critical Care Medicine.

The patients were diagnosed between 2000 and 2002 and were followed for up to seven years, the researchers said. About 5 percent of the patients had XDR-TB.

D’Arcy Richardson of the Seattle-based nonprofit group PATH, which supports public health efforts in about 70 nations, called the findings important. But she noted XDR-TB patients today likely would get more aggressive drug treatment than was given to the patients tracked in this study.

“We have so little information on XDR-TB to begin with,” Richardson, who wrote a commentary with two other TB experts accompanying the study, said in a telephone interview.

Cases of drug-resistant tuberculosis are being recorded around the world at the highest rates ever, with parts of the former Soviet Union especially vulnerable, the U.N. World Health Organization said this year.

Such cases account for about 5 percent of the 9 million new TB cases annually, the WHO said. It said that 489,139 MDR-TB cases emerged in 2006, and about 40,000 were XDR-TB.

There has been scant scientific data on long-term survival rates from XDR-TB.

“We know that it’s a very big problem in Eastern Europe. We know it’s a very big problem in Asia, particularly in India and China, where they don’t necessarily have large percentages of MDR and XDR but because of the size of the population with TB we have significant numbers,” Richardson said.

TB killed 1.7 million people worldwide in 2006, the WHO said. It can be spread by breathing in air droplets from a cough or sneeze of an infected person.