Yes, I think that mental retardation is a disease. Any time there is an affliction that affects an individual due to a mutation in their genetics, it should be considered a disease. I believe that it is even classified as a disease that affects a lot of people at birth.
A disease is defined as any medical impairment of normal functions of an organism not caused by injury. As such, mental retardation is most certainly a disease. The brain function of a person with such an affliction is greatly hindered and thus does not function normally. It is therefore a disease.
Mental disability is sometimes genetic, but it is sometimes the result of brain injury or birth problems. There isn't really a set one thing that is, so it isn't always a disease. It can also be an injury. In many cases there was a lack of oxygen during the birth process.
If you look at the definitions of disease, they are all broadly similar: 'a disorder of structure or function in a human.' Normal human mental ability, across the peoples of the world, generally fits into a narrow range of function. If someone is retarded to the point where they are well outside this normal range, then surely it must be classified as a disease.
No, that is a stupid and unfounded belief. Mental retardation is not a disease. It is a condition that happens due to brain damage, which can occur from physical or chemical injury. It's not something that can be cured. It's also not something that you could catch. Mentally retarded people deserve compassion.