The limericks were all very good. I loved them all equally, but finally managed to narrow them down to a three-way tie:

Mark and Marjorie win for managing to incorporate all the rules (LOTR, psychologist and parasite) and still adhere strictly to the limerick form:

An orc with a raging psychosis
Said “Doctor, what’s your diagnosis?”
The doctor said “Orc,
you eat too much pork.
I think you just have trichinosis.”

Liz (who gave no URL) wins twice for rhyming “sucker” with “fucker” and for calling me a “perfect, post-modernist man” (twice)

A trendy French shrink named Lacan
Thought love added up to rien
But construct or not,
You’re incredibly hot,
My perfect, post-modernist Man.

An earnest young doctor named Beach
fell madly in love with a leech
that little blood-sucker
was such a hot fucker
that heaven seemed quite within reach.

Last but not least, Joeri wins with his tapeworm entry:

A Diphyllobothrium latum
(A pseudophillidean tapeworm)
can grow to 10 meters
among raw fish eaters.
It can make a sturdy man squirm.

Congratulations, everyone.