The third-oldest hospital in the nation, Mass General is celebrating its bicentennial this year. This summer it will open a $579 million, 10-story addition that will increase its bed count by almost 20 percent while adding 19 ORs and a new ED. It is a founding partner of Partners Healthcare System, which has an Aa2 bond rating with Moody’s Investor Services. The major teaching hospital of Harvard University, Mass General has one of the largest hospital-based research budgets in the world. It is the largest non-governmental employer in Boston, with more than 10,000 employees.