Many parasites transmitted on land 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, eggs and larval stages. Larval stages present in intermediate hosts or vectors may also be transmitted passively, but they will be covered separately. Desiccation is a threat to any free-living stage in a terrestrial environment and adaptations to deal with the threat are common.
Coccidian protozoans such as Eimeria are transmitted by a resistant oocyst which contaminates the environment. Waterborne transmission can occur, but the oocyst can resist some exposure to drying.
Relatively few species of digenean transmit completely in terrestrial habitats. Those that do have eggs that must be eaten before hatching; miracidia cannot resist desiccation. Cercariae, which cannot resist desiccation, either encyst in the host that produced them, or stimulate the host snail to surround them with a protective layer of mucus before they are expelled.
Cestodes produce eggs with thick outer membranes that can resist extended periods of drying, and in some cases, freezing.
Acanthocephalan eggs also have thick outer membranes to resist desiccation.
Nematodes transmit via eggs or larvae. If the egg stage is to be ingested by the next host, it is often extremely resistant to environmental conditions, such a with Ascaris.
Some nematode eggs, such as those of Enterobius, are not particularly resistant to environmental stresses. However, they are produced in large numbers, are at least partially embryonated when laid, and usually develop quickly to increase the chance that they will be infective when eaten.
Nematode larvae that hatch from the egg are often capable of entering a dormant state and surviving for months or years when desiccated or frozen. A common adaptation to resist desiccation is for the infective third stage larva (L3) to retain the cuticle of the second stage (L2) as a barrier to moisture loss.
