»commensal

I mis-read the root for this word in Natalie Angier's excellent article on parasites, but the New York Times pounced on my hapless double-click (which I intended to select, so that I could look up "commensal" on-line) and popped up a medical definition: "Of, relating to, or characterized by a symbiotic relationship in which one species is benefited while the other is unaffected." It comes from the Latin "mēnsa" via the Middle English for "to share a meal", apparently.