A couple thoughts.
First, if everyone in town is infected, it will make the PCs job relatively easy. They can start killing indiscriminately. Or just leave. I would suggest making sure that it's only maybe half of the population. That leads to a game of "who can we trust". Which means you need to embed in-game reasons why the PCs must interact with many people. Once they figure out the overall issue, the clues regarding who they spoke to should be evident.
Second, you've got a neat opportunity to leave bread crumbs. If they have a discussion with one infected NPC, you could have another NPC aware of that conversation and refer to it. If you're subtle enough, they won't catch on.
It's an interesting challenge.
Good ideas. I was thinking of having it about half anyways. The players would investigate the murders, get conflicting reports from different npcs with some of the stories not adding up.
It would be neat if maybe some npc's kill infected people, and in turn, the demon has those people killed as well. Gives some nice back and forth (as you mentioned, who can we trust).