The professor was genre-savvy enough to amputate his own arm after it got infected, but while the arm became another walking corpse, it didn't hurt anyone or instinctively attack anyone present. If the bacteria is sentient, does it take on the personality of what the corpse had, (regardless of arms not having personalities)? This would (mostly) explain why the sharks and squids would attack humans every so often, as they do in nature, and why the smaller fish mostly just act like regular animals around people. But then what about when the corpses become just skeletons on the machines?
Which I guess might actually be a bit of Fridge Brilliance, since Walkingcorpse!Kaori whacks Tadashi when he tries to get close to her (showing her anger towards him as a superior other), and impales the professor through the chest (for having plugged her into the machine)!
Pretty much confirmed by around the 18th chapter, on both accounts.
It's shown that after the host on a machine completely decomposes, the machine is set to trap a new one if it passes by.
If the legs are really just a super-intelligent/evolved bacteria, how would bacteria manage to grow that large, and what kind of food source would it feed off of?
Though, given the fact that the legs have a tubular system that fuels them, meaning it could act like a digestive system too? Or maybe Junji just didn't know better.
Rule of Scary. There's no logic whatsoever in this manga, it's all Rule of Scary combined with attempts at making things as grotesque as possible.