Aggressive, drug-resistant staph infections caught in hospitals or from medical treatment are becoming scarcer, another sign of progress in a prevention effort that has become a national public health priority. The decline was seen in a federal study of methicillin-resistant staph, or MRSA. The bug often causes only a boil or skin infection. But researchers in the study focused on invasive cases that can become deadly, invading the bloodstream, flesh, lungs and bones. Researchers found that in nine metro areas, cases of MRSA fell about 16% between 2005 and 2008. That translates to a drop from about 32 cases per 100,000 to 26 cases per 100,000 people. The results suggest aggressive efforts to stop the germ from spreading are working, researchers said.