Mayer-Rokitansky-Küster-Hauser Syndrome. This post may be considered gross by some. You might not want to read it. It's a genetic disease...

... involving your vagina.

The Wikipedia article is pretty sparse on details, so most of what I know about MRKHS was from this article.

Girls with the syndrome are born with no vagina. They often have a normal uterus, ovaries and external secondary sexual organs such as breasts, but cannot have sexual intercourse or give birth.

"Usually the syndrome is diagnosed when they are young and they try to have sexual intercourse for the first time and it hurts," says Marchese.

Women with MRKHS are often embarrassed to talk about it with their parents when they are young and often "live the rest of their lives with no normal sexual life, even though they are normal women with normal feelings", she says.

I think I dated a girl with MRKHS. Every time I tried to... you know... stick it in... she'd tell me that it hurt a lot. She's had other boyfriends before me (and had gone on to have others after me), and she suffered the same with all of her previous boyfriends. So when they say the girls are born with "no vagina", I think that's misleading. I don't know if medical doctors have a special definition for exactly what does and does not constitute a vagina, but this girl did have a hole where one would expect a vagina to be, and in terms of physical appearances, it looked like pretty much like any other vagina I'd seen (as far as I could tell anyway; she was pretty self conscious about her vagina, so she wouldn't let me take a close look at it).