Disclosing
true facts to others would not be defamation (except under the wording of a couple of states'
criminal defamation laws - but these criminal defamation laws are rarely prosecuted because the laws have generally been found to be unconstitutional. . . and this would not apply in Massachusetts, anyway
).
Had the STD information been false, then the communication to others that someone has an STD would probably be considered by the courts in Massachusetts to be defamatory per se.
Even though the disclosure of piedpiper's sexually transmitted disease would not be defamatory under Massachusetts law, because it is true that piedpiper has the STD, this type of disclosure IS covered by Massachusetts invasion of privacy/disclosure of private facts statute (Mass Gen Laws Ch. 214 and 1B), and a suit for invasion of privacy, therefore, is something that could be considered. An individual who has had their privacy invaded has three years to file an invasion of privacy suit, from the date of first disclosure of the facts.
In many states, in order to have a disclosure of private facts privacy invasion action, the disclosure must be made to "the public" and not to just one or a few people. This is not the case in Massachusetts, however. If a private fact is of a highly personal and intimate nature, and the person to whom the fact is disclosed has no legitimate business knowing this fact, then an invasion of privacy action is possible.
There are problems with invasion of privacy actions over the disclosure of facts such as having an STD. A legal action will publicize this fact to a wider audience. There is less risk of the information spreading to others in a shelter, I imagine, than there would be in a publicly open courtroom. In addition, lawsuits are expensive to pursue and any recovery of damages for disclosures that are made to a relatively small "audience" would be limited - and this is if a suit is won. There is never a guarantee of that. Something to think about, at any rate.
You can go over all of the facts with an attorney in your area (one who has experience with invasion of privacy actions would be best), and discuss with this attorney all of the pros and cons of pursuing an action against the shelter staff member who disclosed your STD to a resident at the shelter, should after discussion with the attorney it is shown that the facts could support a suit.
Good luck.