In the heart of Boston, Tufts Medical Center treats scores of health conditions, from administering measles vaccines for children to pioneering next-generation tools that can eradicate the rarest of cancers. But doctors, administrators and other hospital staff struggled to contain a much different kind of epidemic one April morning last year: a wave of thousands of robocalls that spread, like a virus, from one phone line to the next, disrupting communications for hours to come.