The aquatic habitat has a number of properties that affect how parasites find hosts. It provides a reasonably predictable and favorable physical environment for the parasite to survive in while outside the host. Water can transport parasites passively or allow parasites to move within it easily, in three dimensions. Chemical products from their hosts may be transported in water and provide a target for searching parasites. Light is an important cue for many parasites as they position themselves in locations where they are likely to encounter suitable hosts.
The biological properties of aquatic habitats also affect host finding. Diversity of vertebrate and invertebrate hosts is high, even within narrowly-defined feeding and behavioral groups. Consequently, many parasites that rely on passive means of host finding are host generalists. Parasite species with narrow host specificity usually rely on active means of host finding. Indirect life cycles making use of food webs, often with two intermediate hosts, are common.