Host Seth Part I: The Curious Bumps
Seth shows Mom a curious patch of bumps on his stomach. After determining that they don't hurt, Mom ignores the problem until she guilts herself into taking Seth to the doctor. Diagnosis: Warts. Cure: "Beetle Juice," carefully dabbed on every wart with the warning that it will cause blisters, which may be somewhat uncomfortable come nightfall. Turns out "somewhat uncomfortable" actually means "tortuously painful." Seth, through his sobs, exclaims, "I wish I'd NEVER gone to the doctor!" One month later, a few scabs left, Seth prays fervently that the warts never return. If they do, the only way Mom will ever get Seth back in for more Beetle Juice is if he's rendered completely unconsious.
Host Seth Part II: The Curious Lump
In the garage one day, Seth tries to take a small stick away from Caleb, who defends his position by whacking Seth on the head. Seth's crying is a little out of proportion to the minor blow he has just received, so Dad looks more closely at Seth's forehead and discovers that the skin has broken over a curious lump of unknown origin. Caleb did not cause this lump, so Dad brings Seth into the bathroom and proceeds to perform investigative surgery - with Tweezers. Dad discovers a foreign object embedded in Seth's forehead. The skin has grown over it, and Seth's body is in the process of absorbing the object right into his head. Dad pulls, twists, struggles, and eventually wins, extracting a chunk of wood about the size of a large tooth - root and all - out of Seth's forehead.
Host Seth Part III: The Curious Conspiracy
Thanks to Beetle Juice, Tweezers, and Mom and Dad's Astute Mental Prowess, the entire Human Race avoids Certain Destruction. Having not been born yesterday, Mom and Dad are fooled by the Warts and Wood Chunk not at all. They recognize the old Alien-Brain-Disguised-as-Warts-and-When-That-Doesn't-Work-a-Wood-Chunk-Implanting-Itself-into-a-Superior-Human-Host-so-Its-Species-Can-Take-Over-the-World Trick when they see it.
The Aliens clearly don't research their victims well. They have no idea who they're messing with.
Bring it on, Aliens. Mom, Dad, and Superior Human Host Seth are ready for your next move - whatever that may be.