The health insurer Aetna is facing criticism for revealing the HIV status of potentially thousands of customers after it sent out a mailer in which information about ordering prescription HIV drugs was clearly visible through the envelope’s clear window.

For example, in a letter sent to a customer in Brooklyn, the window revealed considerably more than the address. It also showed the beginning of a letter advising the customer about options “when filing prescriptions for HIV Medic … .”

Aetna says approximately 12,000 customers were sent the mailer on July 28 that potentially revealed private medical information, though the company says it isn’t clear exactly how many were impacted, because it depends on how the letter was positioned in the envelope.