I agree with Anika, the term "mood disorder" is completely misleading and technically incorrect, considering that bipolar is a physical illness of the biochemistry of the brain, and so is depression, thank you very much.

Just like diabetes is a physical illness of the biochemistry of the digestive system....
The thing is that many, many, many people do no believe in any mental illness at all. They consider them "fake" illnesses. Or cosmetic illnesses. Or "the illness of the hour." Once a comedian said, "I'm happy today, but yesterday I was sad. I must have bipolar." >.> And the whole "all mentally ill people are a danger to society." That doesn't help, either. There is a lot of stigma. People give the, "just suck it up and get over it," response.
I don't tell people any more. I've never told my dad or my other blood relatives. My husband knows, my friend knows, and my mother-in-law knows. Also I had to tell HR because I was making tons of errors due to rapid cycling, and I tried to get some accomidations on my own. Haha... that's a joke. So now whatever happens where they write me up for something I did wrong which was a clear result of my illness, I take it to my GP and put it in my medicle file. (I can't keep records on my own, so this is close enough....)
But even though my husband knows he doesn't get it. Like last night we were driving through a parking lot. I kept passing spaces because I thought they were turned the wrong way to pull in. He said, "That's because you don't pay attention. Why can't you just pay attention. Something is wrong with you." DUH! Hello, McFly! That's the first time he's actually said "something is wrong with you." Blows my mind....