(CNN) CVS Health is being sued for allegedly revealing the HIV status of 6,000 patients in Ohio.

A federal lawsuit claims CVS mailed letters last year that showed the status of participants in the state's HIV drug assistance program through the envelopes' glassine window.

The complaint, which was filed March 21 in federal court in Ohio, also names Fiserv, the company that CVS hired to mail the letters. On the envelopes used by Fiserv, the patients' HIV status could be seen through the clear window, just above their name and address, the documents states.

The letters included the patients' new benefits cards and information about a mail prescription program.

The companies are being sued by three unidentified plaintiffs, according to the complaint.

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