In the case of a medical, a person will have their health check, but you can refuse them on health grounds if you deem them to be a danger to public health in Canada. Currently, that's defined as those two diseases. As an example, hepatitis may be on the list of 56, but an individual probably would not be refused entry to Canada on health grounds if they were known to have hepatitis, because it's not one of the screening diseases.
On February 16th, 2012. See this statement in context.