ISTANBUL — With cases of a virulent and highly fatal pathogen on the rise, including the first-known occurrence in the United States, epidemiologists and public health officials say some of their biggest concerns about the disease lie in the basic information that they still don't have about it. The virus, called Middle East Respiratory Syndrome, or MERS, is closely related to SARS, which spread rapidly across Asia a decade ago, leaving some 800 people dead. So far, MERS, which has a higher fatality rate than SARS and no known vaccine or cure, has proven to be much less of a
