A Keepsake Christmas Present

Listen up, you absolute simpleton. The joke plays on the idea of giving a gift at Christmas. Normally, when you give something, you don't have it anymore. But herpes is a sexually transmitted disease that you can 'give' (transmit) to someone else, and guess what, you still 'keep' the virus yourself because it's usually chronic. You clearly needed that explained, because you probably only got socks this year, loser.