[Raynor]

In the Silmarillion, "green things fell sick and rotten" after Utumno is built. Also, if I remember correctly, Valinor is as Arda Unmarred would have been, and of Valinor it is said that "it knew no corruption or sickness" (oh well, at least in the begining). The description of Aman in Myths Transformed also gives a similar statement. I believe then that all the (causes of) sicknesses appear first and foremost due to Melkor and his intervention in the Music.