A doctor in New York City who recently returned from treating Ebola patients in Guinea tested positive for the Ebola virus Thursday, becoming the city’s first diagnosed case. The doctor, Craig Spencer, was rushed to Bellevue Hospital on Thursday and placed in isolation while health care workers spread out across the city to trace anyone he might have come into contact with in recent days. A further test will be conducted by the federal Centers for Disease Control to confirm the initial test. While officials have said they expected isolated cases of the disease to arrive in New York eventually,...