First, New York law recognizes "defamation per se" statements, which are statements that are defamatory in and of themselves and have no innocent meaning. Defamation per se covers false accusations of having a "loathesome disease."
STDs are generally considered loathesome and a false accusation of having an STD, therefore, would be defamatory per se.
With false statements that can be categorized as defamatory per se, compensable injury is presumed. You do not have to prove fault or reputational injury in order to be awarded damages in a defamation action. It is automatically assumed that the statement injured your reputation.
Without this presumption of injury, you would need to demonstrate the damaging effects that the false accusations have created (a shunning by others, ridicule, perhaps loss of employment or job opportunity, etc), which can be difficult. You must also show that the rumors were spread with the requisite degree of fault.
It is possible, therefore, that you have a defamation action worth pursuing.
However, herpes is an infection that is relatively common, and there are many kinds of "herpes." Herpes zoster causes chicken pox and shingles, for instance, and a false accusation of having this type of herpes would probably not be defamatory per se or even considered reputationally injurious. False allegations of having oral herpes, likewise, may not be one that presumes reputational injury, although it could potentially be defamatory. This is the virus that causes cold sores, is transmitted through direct contact with the lesion or body fluid of an infected person, does not require sexual contact, and estimates of its prevalence in the American adult population have ranged as high as 80%.
If the false accusations being made about you refer to genital herpes (the virus that causes genital warts), then those statements could cause reputational injury and could be considered defamation per se. The CDC estimates that today one out of four women are infected with genital herpes and one out of eight men are infected with genital herpes. Whether the increased prevalence of this disease decreases the stigma and "loathesomeness" of the disease enough to eliminate its per se presumption, however, is something that I do not know.
Without the presumption of compensable injury going into a lawsuit, you would need to weigh the (generally very high) costs of bringing a defamation suit against the amount of any potential award of damages (and the collectability of any potential award - if your friends are young, without income or assets, you may find it hard to collect damages). There is also the very real possibility of losing your lawsuit entirely (regardless of evidence supporting your case).
You should consult with a lawyer in your area to go over all of the facts, to see if a defamation action makes sense in your situation.
Perhaps just a letter drafted by the attorney and sent to your friends, warning them of legal action that could be pursued against them (if they do not stop defaming you, if they do not retract what was said about you, if they do not delete all defamatory comments posted online, and if they do not refrain from making any further defamatory comments about you) may be enough to stop the defamation.
Good luck.
