In Your Dreams
I don't want to harm you, please believe me of that. I wouldn't dream of harming you. I will grow off of your dreams that you dream every night; I will grow off of your brain energy. But it won't kill you. You humans never use the brain energy anyway, so in a way, I'm just cleaning up liter so to speak.
Perhaps you've had your brain scanned. Or had an MRI. (Well, those are the same things, but you get my idea.) Perhaps your doctor has noticed an abnormality. Sometimes it's a cancerous tumor. Well don't worry--that's not me, and you really should have removed.
But if it's not cancerous, chances are, that lump of abnormal cells is...me. I'm nothing special, really, except that I enter your head while you're sleeping. I'm not a slug, like the Yeerks, or some weird snail or anything like that. I'm like a vapor, one that you can't see and can't feel until I'm in your head.
You'll really feel me through the adjusting period, usually a week or so after entry. You'll have headaches, fatigue, maybe some dizziness or vomiting. You'll think it's a sickness, and eventually it'll pass. The bizarre dreams might last a little bit longer, but eventually they will go away.
And I'll be in your head, watching your dreams, and feeding off of them.