The authors conducted 4 studies suggesting that children attribute different modes of transmission to genetic disorders and contagious illnesses. Study 1 presented preschoolers through 5th graders and adults with "switched-at-birth" scenarios for various disorders. Study 2 presented preschoolers with the same disorders but used contagion links in a contagion context. Studies 3 and 4 presented preschoolers and adults with novel (fictitious) illnesses to determine which cues participants would use to differentiate the modes of transmission. In the presence of kinship cues, children distinguished genetic disorders from contagious illnesses, but in the presence of contagion cues, preschoolers selectively applied contagious links primarily to contagious illnesses. With novel illnesses, preschoolers and adults inferred that permanent illnesses were more likely to be transmitted by birth parents than by contagion. These results suggest that by the preschool years, children recognize that not all disorders are transmitted exclusively through germ contagion.