Last Wednesday, I got bitten by some unidentified insect while playing football. By Friday, half of my right thigh was swollen, red and hard, so I went to see my GP. She had one look at my leg and went:
"Oh dear, you’ve got cellulitis."
I replied: "Thanks for pointing out the obvious, doctor, now can you tell me what’s wrong with my leg?"
She said: "Cellulitis, not cellulite. Cellulitis is a skin infection."
While she was prescribing me antibiotics and urging me to go straight to hospital if it didn’t get better within 24 hours, I wondered what the difference was between the suffixes -ite and -itis. This is what the OED tells us about them:
-itis, suffix. -itis has become in modern medical language, and hence in English, the regular name for affections of particular parts, and especially (though this is not etymological) of inflammatory disease or inflammation of a part: appendicitis, bronchitis, tonsillitis, etc…
So cellulitis is an inflammation of the (skin’s) cells.
When I looked up -ite, suffix, I learned that -ite is the masculine form of -itis, and that both are extensively used in forming names of natural products, diseases, etc. So etymologically, cellulitis and cellulite are the same word, which may mean normal healthy fat or a skin infection. In fact, there is only one French word for both things: cellulite.
Cellulite appeared very recently, in the late 60s, through the American magazine Vogue, or so goes the story. Before then, cellulite was simply known as fat, and was considered a normal feature of adult female skin. This makes me think that the old "conspiracy theory" that says that cellulite was created by the cosmetic industry to turn it into a specific problem to be fixed by buying "specific" expensive products might not be a complete invention.