Bit of a stupid question, but if a yeerk was in a patient taking chemotherapy drugs (or something like digoxin or paclitaxel, which are derived from the toxins in poisonous plants), would it die? I mean, those aren't medications that cross the blood brain barrier, but neither does oatmeal...

1. I'm definitely not a pharmacist, so I couldn't say, other than...

2. Do yeerks cross the blood-brain barrier? Obviously they're in the skull, communicating with the brain. But do they give/receive signals from the host's blood? I don't know of any canon instance of a host being drugged or hormonal or even bleeding heavily and it definitely affecting the yeerk. We know that a concussed host will result in a controller being unable to move around (#52, among others) but I don't know if we ever see a controller get sedated or drunk. It's canon that yeerks feel their host's pain (#31) and temperature (MM4), but those could both just be nerve signals rather than blood-borne ones.

3. You're right about the oatmeal. So it's of course a sci fi device -- artificial maple flavoring somehow being kandrona-like enough to feed yeerks while also giving them some kind of severe deficiency that causes psychosis, sure why not. But if hypothetically something in the processing and packaging is having that effect (maybe it's made in a chee-owned facility? Maybe it's infected with an oat weevil? Maybe it's fumes put off by milk powder?) could mean that there's something faux-radioactive or whatnot about the oatmeal. In that case, maybe there is a signal that travels from the host's stomach or skin directly to the yeerk through some other means than blood. If the yeerks can absorb oatmeal directly through their skin (which no one seems sure about but doesn't want to test in #17) then maybe the oatmeal is reaching the inside of the skull some other way and getting on the yeerk there. I dunno. Also,

4. Can't say about chemo, but radiation treatment for cancer would probably be enough to harm a yeerk. Maybe not enough to kill the yeerk -- you'd probably have to be dosed broadly enough to kill your whole brain for that to happen, which no doctor is going to do -- but maybe enough to inflict some hurt.