if you mean the concept of transness as a disease, it's just from the pathologization of deviance from cisnormativity, one that's perpetuated by the medical system. if we know gender to be a social construct (there is no biological basis for gender, and a very shaky biological basis for sex), and is enforced for the benefit of capitalism (it is in the interest of the bourgeoisie to enforce rigid gender roles and by extension the traditional nuclear family unit, it's productive to generate more workers. the men go out and work, the women stay home with the family, and under the view of reactionary biological determinism, you aren't allowed to change what you were assigned. i recommend the first few chapters of "sexuality and socialism" if you're curious about this), then there's nothing really inherently wrong with deviating from it and identifying as something outside the gender binary or identifying as something different than what you were assigned. but obviously the capitalist system, and by extension the medical system, enforces this social construct by pathologizing that deviance. but don't let that convince you that transness is a disease or affliction. the truth is that it isn't, its just a simple condition of being that comes about because the complexity of human expression does not always fit neatly into the constructed rigid binary. it's just a descriptor for how we interface with the social structure of gender, not something we did wrong or is wrong with us. at least, this is my view on it.