SAN FRANCISCO — The worst forms of the killer tuberculosis bug have been gaining ground in the United States, alarming public health officials over imported drug-resistant strains of a disease that is mostly under control in this country. Although the number of drug-resistant TB cases in the U.S. is small compared to developing nations, health officials here warn that visitors from other countries who are unaware of their infections are bringing over the deadliest mutations. Often those with drug-resistant strains stop taking their medicine when they feel better but aren’t cured. That’s what happened with Pich Chhieng, 61, a teacher who was infected in his native Cambodia and carried it with him to this country. He took medication for eight months but abruptly stopped because he ran out of money and was feeling much better. He didn’t know until he was hospitalized while visiting family in Los Angeles that by neglecting his treatment he had allowed the disease to mutate, and the drug-resistant bacteria had overwhelmed his lungs. ‘I knew it wasn’t cured yet, but I thought it wasn’t that strong,” said Chhieng, who has been forced to stay in California until he is cured. […]