Catchable you mean as in, contagious?  Lou Gehrig's disease or MS isn't "catchable" neither but they are still diseases.

The main point of replacing "gender identity disorder" with gender dysphoria in the DSM-5 was to end the very type of stigma expressed in your comment. Nowhere in the DSM-5 does is say that gender nonconformity is a disease or mental illness. Gender dysphoria refers to the significant stress caused by the discrepancy between one's gender assigned at birth and the gender to which that person identifies. When the level of stress is significant enough to impair normal social and emotional functioning, a diagnosis of gender dysphoria is suitable. Science has yet to produce definitive evidence as to the cause (if any exist) so to declare GD as a choice is pure conjecture and should be dismissed as such.