Hi there, If the blisters tend to come and go, it could possibly be a herpes simplex infection. On the genitals it is usually sexually transmitted, but it can also lay dormant for many years. It is quite possible that he caught the infection before he met you and it just hid in his nerves all these years until his immune system went down. It is also quite possible, though less common, that the oral type of herpes simplex from cold sores was transmitted to his genitals via oral sex. The decreased immunity could be caused by anything, for example a cold, being overtired or even simply high stress levels. You have two options, one is to just wait it out because often the blisters will heal itself as you may have noticed. The second option is to get yourself some antiviral cream or tablets so that the blisters heal faster and he is less infective to you, assuming that you haven t caught it by now. If you would like to confirm it, a simple blood test for the both of you could be done at your local general practitioner s office. Hope this helps, and please let us know if you have further questions. Regards, Dr. Teh