Doctors in San Francisco are seeing the reappearance of Kaposi’s sarcoma—a once-common AIDS-related condition that afflicted roughly one third of patients—among a handful of longtime HIV patients. The doctors do not know why Karposi’s sarcoma (KS) is being seen again in patients.

Kaposi’s sarcoma, a lesion-causing skin cancer, became less prominent in the Western world with the introduction of improved antiretroviral treatment. However, nine cases have been reported in San Francisco between 2004 and 2006 among people diagnosed an average of 18 years ago and who have taken antiretroviral drugs for around seven years.

While none of these cases of KS are severe or untreatable, the reasons for the reoccurrence remains a mystery.