A carcinogen is any substance or agent or combination of substance and angency that causes or contributes to cancer. While the derivation of the word is clear, the word formation itself is a little backwards. Most words using the -gen suffix use the suffix to denote that something originates from the thing, while in this use it denotes causing or contributing to rather than born of or originating from. Carcinogen comes from the Ancient Greek word καρκινος (karkinos) meaning crab or cancer. In both Ancient Greek and Latin, the words καρκινος and cancer were both used to denote three things: crabs, tumors and the constellation still known as Cancer or The Crab. Early doctors noticed that cancerous tumors often resembled crabs, with a central mass and tendrils radiating out from the central body. A carcinologist, however, refers not to cancer doctors but rather the scientist who study crustaceans.
Image of blue crab by Steve Wilde, used with permission under a Creative Commons 3.0 license.
Image of the constellation Cancer courtesy Till Credner at www.allthesky.com, used with permission under a Creative Commons 3.0 license.
Image of cancerous tumor courtesy Emmanualm via wikipedia, used with permission under a Creative Commons 3.0 license.