Before responding, please read this Wikipedia page on the
Leucochloridium paradoxum - Wikipedia. A few responses so far don't seem to understand exactly what this organism's lifecycle really entails.
Many parasitic organisms have complex lifecycles; in some cases they are dependent on multiple species for their reproductive cycle.
Take for example, Leucochloridium paradoxum. It's a parasitic organism that reproduces via eggs inside of birds. Then the fecal matter containing the eggs are consumed by snails. The eggs hatch inside the snail and then proceed to develop into a sporocyst with various branches throughout the snail's body. The ends of the branches then develop into a broodsac which infest the snail's eyestalk.
In a highly disturbing aspect of mimicry, the broodsacs will pulsate to attract predation by birds while simultaneously mind-controlling the snail:
(Look up "Leucochloridium" on Youtube if you want to see this in action. Warning: it's a bit gross.)
Birds will then attack the snail, ripping out the broodsac from the eyestalk and then the whole cycle starts over.
This is obviously a highly complicated lifecycle that clearly defies biological evolution. Therefore it must have involved the hand of an intelligent designer.
That said, how and why would an intelligent designer create such an organism? Imagine having a parasite in your eyeball that forces you wait around until it's pecked out by a bird. What purpose does such a parasite serve?