If we define a disease as an alteration of the health, of the functions of living beings, then trisomy 21 is indeed a disease since the presence of a supernumerary chromosome 21 leads to organic and functional alterations whose association and evolution are characteristic.

On the other hand, the term disease implies the possibility of medical intervention and treatment, which tends to give a hopeful vision of Down Syndrome. This is why Jérôme Lejeune used the term “disease of the intelligence”.