Note that the USPS also scans and stores images of the front and back of all mail it delivers, even if you don't opt in for their digital delivery service[0]. So this means that the USPS - and anybody else with access to their images, which includes law enforcement - now knows the HIV status of these people.
Scarier point: USPS probably knows without that. If these letters are sent out in batches and the envelopes/timing/vender is unique, then all it takes is knowing that 1 letter is abut HIV and then you know all of them are without ever seeing the content.
[0] http://www.nytimes.com/2013/08/03/us/postal-service-confirms...