sentences:  In 2013, the Mexico–United States border was noted to be "a very hot region for drug resistant TB", though the number of cases remained small.It has been known for many years that INH-resistant TB is less virulent in guinea pigs, and the epidemiological evidence is that MDR strains of TB do not dominate naturally.A study in Los Angeles, California, found that only 6% of cases of MDR-TB were clustered.