Here's an article from the Morgellons Watch site regarding disease classification:
"In any discussion, particularly in science, the defining of terms is vital to progress. If you think a term means one thing, and someone else thinks it means something else, then unless you address this difference, you are not going to come to any meaningful agreement.
This problem is particularly apparent in the phenomenon of “Morgellons“. There are two particular terms that cause problems. The first is, obviously, “Morgellons” itself, and the second is “Delusional Parasitosis” (DP). The confusion between these terms has led to the doubly misunderstood (and hence incorrect) statement that “Morgellons is DP”. Morgellons is not DP."
Read the rest of the article here