In retrospect, it definitely has to be either some sort of nutrient mix, or maybe additional doses of viral culture (maybe humans actually do generate antibodies that need to be periodically overwhelmed with re-exposure). But it had me wondering why Zosia would be drinking milk in the hospital while also hooked up to an IV, it’s not as if she needed the fluids from it.

Could be that this is what most everyone eats now, and the “coffee” we see her drink on her own in episode 2 is just more of this stuff. It had me wondering at that time if the afflicted would actually still bother wanting to enjoy food and drink like people do, or if they opted for absolute pragmatism and bare essentials only. Seems like the latter may indeed be the case.