The Public Health Agency of Canada (PHAC) has tightened security of its Level-4 National Microbiology Lab (NML) in Winnipeg years after two scientists were escorted out over concerns about security breaches . But experts worry the back door may still be open to graduate and postdoctoral students affiliated with the University of Manitoba . Some of the NML scientists also have appointments at U of M, which means some of their students have access to the country's highest-security infectious disease lab .