By definition, maybe.
A disease is an abnormal condition affecting the body of an organism, often constured to be a medical condition associated with specific symptoms.
So it could be, because it does affect the organism, and possibly the brain (I'm not up on the latest medical research into it, so I don't know if it's been proven that the brain chemistry is significantly altered); and it does have specific signs and symptoms.
But it could not be, because it affects the mind, and is thus in that weird limbo of conditions which aren't technically covered by medical definitions which are obssessed with finding a problem in the physical form. Sometimes it is only the metaphysical which is affected, in this case the mind but not the brain.
If you're asking should autism be treated medically, and should people who have it be afforded treatment and care as if they had a physical problem requiring care (I can't think of any equivalents right now, but I know there would be some) then I think yes.
I am also high functioning autistic, but I didn't know until I was about sixteen, when I went into therapy for entirely unrelated reasons, and it ended up coming out along with a whole bunch of other stuff.
Looking back it seems obvious that I had autism, because all of the signs were there, analytic, problems connecting emotionally with other people, problems with understanding problems when they didn't relate directly to me, and my mum confessed that she had often thought about having me checked, so I could receive help if needed, but I got lucky with high functioning autism and thus was good at learning behaviours if nothing else. I might not have understood why I should act a certain way in a certain situation, but I knew that I had to so I did.
Speaking from my own experiences, had I known earlier, and had someone who could tell me what it would be like, and especially that it wasn't something that was wrong with me, which was what I thought for a long time, it might have made things easier. Growing up thinking you're different to everyone is one thing, but growing up with the nagging sensation that you are not like other people, and having it reiterated to you every day becomes oppressive and disheartening. I was a much better person when I knew and was receiving help.