Is it passed down genetically
There is considerable evidence to suggest that yes, it is, at least in part.
For years (since around the 1950s), psychologists and psychiatrists have labeled that part of the personality which is (likely) genetic as the temperament, and there are nine key facets that make up the temperament. One of these facets, called Initial Reaction, deals with whether someone will deal with new environments or people in a positive or negative way. In a negative reaction, an infant, for instance, will shy away from a stranger, grab its mother for reassurance, or carefully study a situation, whereas the infant with a positive reaction will boldly approach.
So if that part of the temperament forms the basis for how shy we are, and if the temperament itself is formed by our genetic makeup, then shyness can be said to be genetic. Not exactly a disease, however.
Of course, this is still just conjecture and theory, as it can never be positively proven, but over the last half century, the evidence has been steadily mounting.
No one ever suggested that you can't learn to be more sociable, dawnathen, but your innate level of shyness is most likely determined long before you were born. Now, being as these are psychological traits and not physical, they can be altered to some extent, but the simple fact is some people are born to be more shy than others.
And unfortunately, your becoming more outgoing as you got older doesn't exactly present enough evidence to counteract 50 years of intense psychological scientific research. :) It's really quite interesting to read about since it flies in the face of everything we tell ourselves. "I can be anyone I want to be!" is a very appealing notion. Well, the evidence tells a different story. Not everyone is born with the charisma needed to be a leader, not everyone has the bold curiosity to be a good scientist, and so on.
Read up on it and take a few more psych classes. There are some amazing studies on this subject.
I was shy in high school and I am very sociable now. Its not genetic, I'm tellin ya. It's also who you're around. There are people I can go totally crazy around and others I just sit in the corner. Nobody in my family is like that. I don't think I'd call it a disease.
EDIT -- Here's some studies on the genetic nature of temperament for further reading:
No, you meant it's not genetic. That's why you said, and I quote:
"Its not genetic, I'm tellin ya."
It's cool. No need for backpedaling.
Like mikeh said, there are studies on temperament that show it is indeed genetic... it is not something you can simply "get over".
All I meant was it's not a "disease". shyness is not a disease.
No, it's a personality trait.
Dude, get over it.