I'm still building the whole quarantine incident in my head so it's subject to change but as I see it so far he was a custodian at the hospice where Coeus conducted the experiment that got out of hand (oopsy). This was all done covertly so to him Coeus was pretty much a normal staff member there, he most likely didn't even know his real name but he would say hello to him each time they crossed path. He was the kind of social but not overbearing person, the type that won't go elsewhere if there's someone already smoking behind the building but won't engage in needless small talk either, just a very calm, laid-back individual. They were not close-close but I think Coeus ended up liking the familiarity of their very small interactions, like a little gesture you don't realize you like until it isn't there, and he likewise enjoyed the fun little enigma Coeus was in his day-to-day (he kind of quickly figured that a doctor that only visits and never stays on site very long was weird, but he never explicitly told Coeus if he suspected him of being responsible of what got them quarantined).

Coeus was supposed to work only on a little group of patients there so he truly was collateral damage of the outbreak. As for the duration, it was a little less than half a year before Coeus was plucked from there -by then he was obviously the only one left alive and tbh, if they had come even a few days after he might have succumbed to spesis.