<An invasion. The Yeerks. They are parasites.>

And then everyone gets a picture of -

A greenish-gray slug, large for a slug but easily pocket-sized.

<They need hosts. They enter through the ear. Without them they are almost powerless, but when they become Controllers - when they wrap around a host brain and take over their thoughts and actions - they are dangerous. They prefer cooperation. They do not need it. We had hoped to stop them, but soon they will be here to obliterate all trace of me and my ship.>