Frankly, they didn’t know what to do. First, they didn’t know that a historic plague was starting; it could have been a smaller epidemic of the sort that passed through every few years. So they began by not altering their routines and hoping for the best.
They did sick care for family members until many of them started panicking. Then they abandoned the sick, sometimes leaving them with a pitcher of water and promises, locking the door and not coming back until it was over. Survivors who wrote about the terrible time recalled instances of abandoned husbands, wives, and young children.
Nobody really thought about quarantining the sick, but a few healthy people tried their own quarantine. One family in Italy filled the house with supplies and then locked the doors and sealed the windows. They just didn’t come out for several months, and they survived. The rich fled into the country, assuming that the plague would not follow them there. They felt safer in the hills, where the air was cleaner and cooler. Of course, if this strategy worked, it was only because it acted as quarantine. The plague spread inland and wiped out country villages like anyone else.
The Pope stayed indoors next to a hot fire as the infection passed by in 1347 and 1348. It worked; the Pope survived. Maybe it was too hot for the fleas.
Natural selection pushed those with natural immunity into posts of public service, including grave digging. Communities could hardly keep up with the pace of grave digging, and at the peak of the infection they resorted to mass graves. Bodies were layered with sand between them, “like lasagna,” one Italian commented. Grave diggers didn’t seem to get sick, so with flawless post hoc reasoning, some people concluded that breathing the smell of the dead caused immunity. Parties of terrified citizens would go out to the burial grounds and deeply breathe the foul air of the stacked corpses.
By 1349, as the plague roared into Northern Europe, people tried public confession of sin. Flagellants vowed to go on pilgrimage (locally) for 40 days, walking in a group and lashing themselves with whips in each town center to show public sorrow for sin. They hoped that God’s punishment could be turned away.
The plague went into Muslim areas of Spain, and we have records that religious fatalism wasn’t very helpful. Doctors who tried too hard to save patients might be flogged for thwarting God’s will. So some people really did nothing when the plague arrived. They waited it out and cleaned up afterward. Christians believed it was meritorious to save the sick, perhaps modeling their efforts after their cult of the saints. They just didn’t know how. It would take several more visitations of the Bubonic Plague before anyone began to notice isolation and masks helped.