UPDATE: A federal class action lawsuit has been filed against Aetna. Scroll down for more details.

Nearly 12,000 people taking HIV meds received a letter from insurer Aetna Inc. in which information about their prescriptions was visible in the envelope’s window, Bloomberg reports. Specifically, the beginning of the letter informing patients about options for filling their HIV meds could be seen. Some recipients are HIV positive; others are taking the HIV prevention pill Truvada as pre-exposure prophylaxis (PrEP).

“Aetna’s privacy violation devastated people whose neighbors and family learned their intimate health information. They also were shocked that their health insurer would utterly disregard their privacy rights,” said Sally Friedman, legal director of the Legal Action Center in New York City, in a press release. She is working with Ronda B. Goldfein, executive director of the AIDS Law Project of Pennsylvania, in calling for an end to the letters in the current form and for Aetna to develop a plan to correct its practices and procedures.

The two lawyers sent a letter to Aetna that begins, in part: We are writing “on behalf of individuals in Arizona, California, Georgia, Illinois, New Jersey, New York, Ohio, Pennsylvania and Washington, DC, to demand that Aetna immediately cease and desist from breaching their privacy by sending mail that illegally discloses that they are taking HIV medication. We also are demanding that Aetna take corrective measures to ensure that this gross breach of privacy and confidentiality never reoccurs.”

People taking HIV meds can face serious issues of stigma, Goldfein noted, adding that a violation of that privacy “creates a tangible risk of violence, discrimination and other trauma.”

Aetna sent a letter to its customers notifying them of the breach and apologizing, according to Bloomberg. “This type of mistake is unacceptable,” Aetna said. “We sincerely apologize to those affected by a mailing issue that inadvertently exposed the personal health information of some Aetna members.”

UPDATE: A lawsuit was filed Monday, August 28, demanding that Aetna cease the practice, reform procedures and pay damages, according to a press release posted on the AIDS Law Project of Pennsylvania website.

The lead plaintiff is a 52-year-old Bucks County, Pennsylvania, man who claims his sister learned from an unopened envelop that he was taking HIV meds. In this case, he was taking Truvada as PrEP.

The lawsuit was filed in the U.S. District Court for the Eastern District of Pennsylvania. Aetna is based in Blue Bell, Pennsylvania, and Hartford, Connecticut. In addition to the AIDS Law Project of Pennsylvania and the New York–based Legal Action Center, the lawsuit was filed along with Philadelphia law firm Berg & Montague, PC.