Many parasites transmitted through water find a host by simply waiting for a host to find them. The parasite has no direct control over the host it will enter. This mode of transmission applies to cysts and some egg or larval stages. Larval stages present in intermediate hosts or vectors may also be transmitted passively, but they will be covered separately.
Cysts are produced by many protozoans and pass in the feces of their host. The hosts may be terrestrial or aquatic but the usual mode of transmission is that the fecal material and cysts contaminate the water supply.
Entamoeba produces waterborne cysts. Each cyst contains a multinucleate (N) trophozoite that will divide soon after entering the new host. The cysts are thin-walled and will not resist desiccation. They can survive in water for a few weeks.
Giardia produces waterborne cysts. Each cyst contains a partially divided trophozoite that will complete division into two organisms following excystment in the new host. They can survive in water for a few weeks. Two pairs (S1 and S2) of sucking disks can be seen here.
Many eggs of parasites in aquatic environments hatch and release free-swimming larval stages. Some, however, must be eaten. This applies to acanthocephalans, cyclophyllidean cestodes, and some digeneans and nematodes. Typically, eggs produced by aquatic parasites have relatively thin outer membranes because they do not need to protect the parasite against desiccation.
Many cestodes produce an egg from which a coracidium larva hatches. This is a free-swimming stage, using its ciliated external layer (C), but it doesn't swim to seek out hosts. Its swimming simply positions it in the water column where it is likely to be eaten by a copepod intermediate host. Then the enclosed oncosphere (O) infects the copepod. Coracidia will only live a few days.
