What can I ask the PCs to sacrifice to save people (NPCs) from a plague?

DND 5.0 ed

New on 5.0, played the 3.5 edition. Inside my new campagne the PCs are followed by a big plague. They are immune and they have a stone that can save the sick people that, otherwise NPC will die. In some way the plague will follow the PCs so they are, in some way, responsabile of the poeple who get sick. Using the stone they can heal people, but I want they do a moral decision in order to sacrifice some of their life to save people, if they want.

What can I ask the PCs to sacrifice to save people (NPCs) from a plague? The only things that I have in mind is a long curse like -1 to all hit roll for a week or, more strong, heal people will cost some EXP points. (at start only few, but any time they will us ethis power ther spend more exp points)

Do you have better ideas? How can I balance this things?