ANN ARBOR – Michigan Medicine has partnered with about 100 of the country’s top health care systems urging Americans to mask up as COVID-19 cases reach record-breaking highs. Over the past two weeks, more than 900 medical workers at Mayo Clinic tested positive for COVID-19. The message reads:“As the top nationally-ranked hospitals, we know it’s tough that we all need to do our part and keep wearing masks. However now is exactly the wrong time to let up,” Marschall S. Runge, CEO of Michigan Medicine and dean of the U-M Medical School said in a statement. “But for now, we have to use the tools that we know work: wearing masks, staying socially distant and washing hands.