Major insurer Aetna is facing a class-action lawsuit after it inadvertently revealed that about 12,000 of its customers have HIV.

A 52-year-old man in Bucks County, Pa., is suing the insurer because his sister learned that he had HIV by looking at an unopened envelope sent by the insurer. Information showing that he was taking HIV medications could be seen through the large window on the envelope, according to a statement from the AIDS Law Project of Pennsylvania, which filed the lawsuit Monday.

The same type of envelope was used to mail sensitive information to about 12,000 customers in at least 23 states, the lawsuit said.

Aetna last week apologized to "those affected by a mailing issue that inadvertently exposed the personal health information of some Aetna members. This type of mistake is unacceptable and we are undertaking a full review of our processes to ensure something like this never happens again."

The company did not comment on the lawsuit.