It's a plug in thing (light socket) that you use for babies that emits menthol. We put a few in the
bathroom (has to be a small room), ran the shower
on hot, and left it plugged in. They actually seemed relieved, breathed better, and got some rest.
I hope your last cat makes it through this. It's a good sign he's still alive, as it means he's probably in good health (before this, that is). If he hasn't gotten his shots yet, after this do so. Have him wormed and the whole deal.