How did the chicken get in chickenpox?

It is not clear where the chicken in chickenpox came from. It might be assumed that it comes from the farmyard fowl of the same name, in which case it has a Teutonic ancestry in the Old English 'cicen' and Middle High German 'Kuchen'. It has also been suggested that the word was derived in the 16th century from the chick-pea, a dwarf pulse. If this is the case then it is not a fowl word at all but rather, has a French derivation in 'pois chicke', which is turn derived from the Latin word for chick-pea, 'cicer'. There is no way of determining whether either of these possible derivations is correct; there is no word for varicella which resembles chickenpox in any other language.

From: Australian Herpes Management Forum