An important transition is happening across the United States: Cancer was the leading cause of death in more counties in 2015 than 13 years earlier, a new study finds. However, the opposite was true for heart disease during that period; fewer counties reported it as the top killer. In fact, cancer will replace heart disease as the leading cause of death in the United States within two years, according to US Centers for Disease Control and Prevention projections referenced in the study, published Monday in the medical journal Annals of Internal Medicine. “We’re just on the cusp of the transition from heart disease to cancer as the leading cause of death,” said Dr. Latha Palaniappan, lead author of the study and an internist, professor and clinical researcher at Stanford University Medical Center. A decades-old theory described a shift that occurred in health and disease patterns in the United States during the last century. Early in the 100-year period, infectious diseases including tuberculosis, diphtheria and flu took more American lives than other illnesses. Yet by the end of the century, chronic diseases, including heart disease and cancer, had become the leading cause of death. The theory described these complex patterns and suggested that such shifts stemmed from economic and social conditions.Recent data suggests that the nation is experiencing a new transition, this time within the chronic disease category itself. To understand these changes, Stanford Medical School researchers examined more than 32 million death records across 3,143 American counties for 2003 through 2015. The research team looked not only at medical information but at demographic data, including income and race.