Doctors say a woman in Michigan contracted Covid-19 and died last fall two months after receiving a tainted double-lung transplant from a donor who turned out to harbor the virus that causes the disease — despite showing no signs of illness and initially testing negative. Officials at the University of Michigan Medical School suggested it may be the first proven case of Covid-19 in the U.S. in which the virus was transmitted via an organ transplant. A surgeon who handled the donor lungs was also infected with the virus and fell ill but later recovered. The incident appears to be isolated — the only confirmed case among nearly 40,000 transplants in 2020. But it has led to calls for more thorough testing of lung transplant donors, with samples taken from deep within the donor lungs as well as the nose and throat, said Dr. Daniel Kaul, director of Michigan Medicine’s transplant infectious disease service.