A new study challenges the prevailing wisdom that high rates of hepatitis C virus (HCV) among members of the baby boomer generation are a result of their youthful experimentation with drugs, unsafe tattooing and risky sex, aidsmap reports.

Publishing their findings in The Lancet Infectious Diseases, researchers analyzed 45,316 sequences of genotype 1a of hepatitis C, the dominant strain in North America.

The scientists concluded that hep C saw its greatest infection rates between 1940 and 1965. These rates tapered at the end of this period and plateaued between 1965 and 1989. The 1990s saw falling infection rates; starting in 2000, the rates rose modestly.

These patterns suggest that, in fact, unsafe medical procedures are the root cause of the virus’s spread among baby boomers. This generation was too young during the period of the epidemic’s vast expansion for members to have largely contracted the virus from behaviors—including injection drug use—that are more common among young adults.

Furthermore, the post–World War II era saw a great upswing in the number of medical procedures. Proper safety measures, such as the introduction of disposable syringes during the 1950s, were slower to come onto the scene.

Blood transfusions were likely largely responsible for HCV transmissions between 1965 and 1989, given that thorough screening for hep C in the blood supply did not begin until 1992.