I find it odd that no creature type is immune to the "nauseated" and "sickened" conditions. One would think that, at a minimum, constructs and undead should be, being nonliving. By the usual rules of English construction, being immune to disease should make one impossible to sicken, but as "sicken" is a term of art that isn't the case by RAW. The use of the word "stomach" in the 3.5 SRD description of "nauseated" lends support to the idea that constructs and undead, lacking stomachs, can't be nauseated, but it doesn't actually say that lack of a stomach grants immunity to the condition.