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There are some words that just because they are used a certain way so often they end up being acceptable, although they are not technically correct. That happens with medical plurals quite a bit.
Technically, the plural of cellulitis is cellulites.
You may still have to check with your account specifics or client to see how “technical” they want to be as far as accepting common usages or only going with the dictionary answers.
This is one of the few dictionaries that always lists plurals of words (even the ones that only add an “s”) and if there is more than one acceptable plural.
http://www.memidex.com/cellulitis