Lungworm
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.