Something like 70% of the population has cold sores, or HSV1. It's the oral version of herpes (the other version is HSV2, and can be anywhere, but is usually associated with genital herpes). You probably never HAD the virus, and recently caught it from kissing or eating after someone. It's also possible that you had it as a child, and your immune system kept outbreaks in check for a few decades, but something strained it, and now you're showing symptoms.
There are tons of meds to treat the symptoms, but there are also suppressants you can get with a prescription.
This is why I don't drink after people.. That and general paranoia over hygiene.