The development of knowledge of germs and viruses in relation to AIDS and flu was examined in a predominantly Mexican American sample of children aged 8-9, 10-11, and 12-13. Children progressed with age from identifying the disease agent for these diseases as a nondescript germ or something other than a germ to implicating a disease-specific germ or virus. Parallel age trends in mastery of the two diseases were observed; gender and ethnic differences were minimal. Solid command of germ and virus concepts in relation to AIDS was associated with more causally sophisticated understanding of the disease but not with more accurate knowledge of modes of HIV transmission. Grasp of flu germ/virus concepts did not contribute to greater understanding or knowledge of AIDS. Overall, children seem predisposed to construct a germ theory of an unfamiliar disease such as AIDS but need help in differentiating between one germ and another.