Shope, Richard Edwin (1901–1966), American pathologist. Shope was the first to isolate an influenza virus, the first to establish the feasibility of animal immunization against influenza, and the first to explain the cause of the pandemic of Spanish influenza following World War I. During the 1920s and 1930s he demonstrated that pseudorabies is caused by a virus for which pigs act as intermediate hosts. In 1933, while studying tumors in cottontail rabbits, he discovered a virus causing similar growths in domestic rabbits that can eventually become malignant. This discovery marked the first time that a potentially cancerous tumor was shown to be caused by a virus.