Lungworm

The lungworm is a type of parasite. It lives inside other animals' bodies. Lungworms are roundworms in the order Strongylida. They have round bodies. When people say the word "lungworm," they may mean any of a few worms: "Eucoleus aerophilus", "Oslerus osleri", "Crenostoma vulpis", "Eucoleus boehmi", or "Filaroides hirthi". Some people also use it for "Angiostrongylus vasorum", but that worm is really a heartworm.
Lungworms can live in foxes, dogs, cats, sheep, and other animals. Some lungworms live in the animal's nose. Some live in its throat. Some live in its lungs.
In sheep, lungworm is not as dangerous as other worms.
Life cycle.
Some lungworms can send their eggs from one host to another. Other lungworms need another host in between to grow.
Snails and slugs eat lungworm eggs. The eggs grow into larvae inside the snail or slug. When a fox or dog eats the snail or slug (or another animal that just ate the snail or slug), it catches lungworms. The lungworms lay eggs in the host's lungs, then it coughs them up to its throat and swallows them. The eggs leave the host in its feces. Then the snails or slugs come to eat the feces and take in the eggs too. In some lungworms, the eggs go to new hosts when an old host coughs on them.
In popular culture.
Lungworms in sheep are part of the story of the 20th century book "Independent People" ("Sjálfstætt fólk") by Icelandic author Halldór Laxness.