Huntington's chorea, usually called Huntington's disease, is a hereditary, degenerative disorder of the central nervous system. Symptoms don't usually appear until mid-life (40s or 50s). Chorea develops because of the destruction of brain cells.
In Sydenham chorea, the chorea comes from an autoimmune reaction after a streptococcal infection. It usually affects children, may range from mild to severe, and eventually goes away.