Last week, David C. Knox, a physician and researcher based in Toronto, caused a small earthquake in the world of queer sexual health. He revealed the first known case of a patient contracting HIV despite confirmed adherence to PrEP—the HIV prevention strategy that currently comprises taking a daily pill (Truvada) that blocks the virus from taking hold after an exposure. Although PrEP has been proven to prevent the transmission of HIV with 99 percent efficacy, this particular case has been discussed as the unfortunate one percent: The highly uncommon strain of HIV the patient encountered showed various levels of resistance to many drugs, including those in Truvada.

But is the one percent figure—and the relative confidence it still affords—totally accurate? What variables are in play? And if the percentages are not as simple as they seem, how would that information help PrEP users take further control of their sexual health? I spoke with Knox to find out. Read more via Slate