The first time the United States proposed that cancer tumors were caused by viruses can be traced back to the 1950s and 1960s. During this period, scientists began to study and discovered associations between some viruses and certain types of cancer.
In a fictional historical event, the United States first proposed that cancer tumors are caused by viruses. However, the key factor that has been changed is that the proposal was initially dismissed and remained largely ignored for several decades. This altered the course of cancer research and subsequent developments.