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House (Hugh Laurie) faces a possible lawsuit after a confrontation with a gay "stalker" named Kalvin (Matthew John Armstrong). It is soon established that Kalvin is a victim of AIDS, but his symptoms are most confusing--as are those suffered by the patient's father (Wings Hauser). The situation reaches the crisis stage when Kalvin coughs blood all over Cameron (Jennifer Morrison), who may become fatally infected as a result. Elsewhere, House finds a rather unorthodox way to spend time with Stacy (Sela Ward) despite her decision to remain with her husband--and his strategy involves "tampering" with Stacy's pet rat!