A woman I see often at school fund-raisers is notorious for losing her glass and drinking other people's wine. At the last one she had an obvious cold sore on her lip. I suggested she mark her glass so she would know which one was hers and she glared and stalked off. Is there some other way of handling this?
First, a disclosure: I have form in this area. No, not gratuitously offending fellow parents at the Junior Athletics Auxiliary Trivia Night, although I'm sure I've done that, too. I mean inadvertently picking up other people's glasses at parties. I have done this so often it is a near-miracle I have contracted nothing worse than a nasty bout of buyer's remorse after the silent auction - weekend for two in Woop Woop, anyone?
Despite coming at this question from the moral low ground I don't wish to make light of your hygiene concerns; quite a few infections are capable of spreading themselves via shared drinking vessels.
Rather than singling out the offending individual, leper-style, consider writing every guest's name on his or her glass with a felt-tip pen. This is admittedly inelegant and not something you would do at anything other than a very casual affair. It also strikes me as slightly infantile, but it's arguably better than having the likes of me making off with your merlot. Or, you could buy any number of identifying baubles to sling around the stem of a glass: there are versions that stick to the bowl as well. Either kind are often sold as "fun", for which you may read "twee". If miniature tiaras, stilettos and sparkly beads are not your thing, you may elect to embrace your inner Tonia Todman and make your own. Perhaps featuring the school crest? If your kids' school doesn't already have a weighty-sounding motto, hop on to Google Translate and procure yourself a gloriously inaccurate Latin rendition of "if you drink my wine I will kill you".
I add the caveat that all these precautions count for nothing in the face of a particularly well-refreshed party guest struggling to remember his or her own name. If you want your glass to go unmolested, make like Gollum and the ring - keep it close or suffer the consequences.