A surge of positive COVID-19 test results has Public Health Madison and Dane County sending a disturbing message:  If you live or work in downtown Madison, just assume you’ve been exposed to the virus.  Health officials are urging those people to start monitoring themselves for symptoms.  More than 900 cases were confirmed in Dane County in the last week alone and 70 percent of them involved U-W students or staff members.  Doctors point out that the university community isn’t an island and members interact with the whole city.