From the Wikipedia entry for Ranunculus:

 Quote:
The term sardonic (sardanios), "bitter or scornful laughter", is often cited as deriving from the name of the Sardinian plant Ranunculus sardous, known as either σαρδάνη (sardanē) or σαρδόνιον (sardonion). When eaten, it would cause the eater's face to contort in a look resembling scorn (generally followed by death). It might also be related to σαίρω (sairō) "I grin".


Rabid, apoplectic, and hysterical have come to mind as “other words from illnesses that aren't used solely in descriptions of them”. There is some disagreement as to which physical conditions and phenomena deserve to be classified as diseases or disease manifestations.