Sounds like a medical procedure.

Cathexis: After a shuttle is attacked, Chakotay is left brain dead. Before long, the crew begins to suspect there may be an intruder on board.

And right after a Holodeck disaster episode, Janeway is taking part in a holonovel. I hope they ran plenty of diagnostics.

Chakotay's told B'Elanna all about how to use the medicine wheel. Apparently such knowledge is also incorporated into Star Fleet medical data-banks, as the HoloDoc knows about them too.

Nice how the bridge staff can be locked out from vital functions such as navigation, no matter how temporarily. Could cause a lot of havoc mid-battle.

Yaaaaaaaaaay! The alien could be in anyone! Paranoia a go go. And Janeway transfers all the command codes to HoloDoc.

"Neelix, just because a man changes his drink order doesn't mean he's possessed by an alien." Ha!

Tuvok was evil all along! *gasp*

Lucky they could retrieve the warp core, considering they only have the one... And as a bonus HoloDoc gets Chakotay back into his body none the worse for wear.