MARACAY, Venezuela—A string of deaths in a hospital here has sparked fears of a potent, mosquito-borne disease and led authorities to seek a doctor's arrest for allegedly sowing panic, leaving residents wondering how to explain their symptoms.

Angel Sarmiento, president of the College of Doctors in Aragua state, told reporters on Sept. 11 that a virus or bacteria may have been responsible for the deaths of eight patients in quick succession at the Central Hospital of Maracay. A ninth patient died three days after Dr. Sarmiento's...