The infectious disease tuberculosis (TB) is becoming increasingly resistant to medical treatments in Russia—and in a way that has surprised scientists. Almost 150,000 cases of TB were recorded in the nation in 2012, according to the World Health Organization. Of those, about 18,400 were a multi-drug resistant form of the disease.

A study published in the journal Nature Genetics has challenged the common theory that improper use of antibiotics is the major factor in the bacterium’s growing strength. After sequencing the TB genome from 1,000 infected Russian patients, researchers found that certain natural processes with the bacterium also result in specific strains resisting drug treatments and spreading more quickly.

Scientists who conducted the study found that many TB strains are morphing through what are called “compensatory mutations.” When a strain comes into contact with antibiotics, it works to develop a resistance—a process that leaves the bacterium weakened. The TB then goes through an additional “compensatory mutation” of its own, which allows the now-drug-resistant strain to “compensate” and return to its previous virulence.

Nature magazine reported on the study: “Nearly half of the TB isolates [samples] were multi-drug resistant, which means that they were impervious to the two common first-line antibiotics that cure most TB infections, while 16% of these isolates also harboured mutations that made them impervious to ‘second-line’ drugs. These infections are more expensive to treat, and patients who receive ineffective drugs are more likely to spread TB.”

The World Health Organization found that about 450,000 people developed multi-drug resistant TB in 2012, with the majority of those cases in India, China and Russia.