The global spread of parasites threatens native animals even in the most isolated of all habitats on Earth. In the Hawaiian archipelago, the five native species of stream fishes are infected with several species of alien parasites introduced through human activities. These exotic parasites have been shown to present greater disease threats to their Hawaiian fish hosts than do parasites that have colonized native fish by natural means. By studying the biological characteristics and mechanisms of transmission of parasite colonists, researchers can enhance understanding of the parasites' global dispersal.