Nausea derives from Latin and is the plural of nauseum. It has the same root as natation, the sense or act of swimming. The singular form of the word appears in English in phrases such as: "He talked about life insurance ad nauseam", implying the speaker has said more than the listener wants to hear. Since nausea is already plural, it is not correct to write nauseas, for the same reason it is not correct to write datas, medias, or magmas.
It is also the name of a novel by Jean Paul Sartre.