A class-action lawsuit was filed yesterday on behalf of residents of Libby, Mont., against W.R. Grace & Co., former owner of an asbestos-contaminated vermiculite mine that operated for more than 60 years in the town and is alleged to have caused serious health problems for community members. Libby, population 2,700, is potentially the site of one of the nation’s worst public health disasters in many years. Federal investigators say nearly 200 deaths have been caused by asbestos and more than 300 other cases of asbestos-related diseases have been diagnosed. No one knows how high the numbers could go. The case of Helen Bundrocks is, unfortunately, not unique: She watched her husband, who had worked in the mine, die of asbestosis, then she was diagnosed with it, then four of her five grown children, none of whom worked in the mine, were diagnosed with it as well.