Healers setting up shrines or "hospital" tents would have been my preferred route, but I think 0 A.D. is going a little more metaphorical with its healers (like in AOK). They'll pretty much function similarly to the monks in AOK (AOEII), plus temples will heal units like in Age of Mythology (Apollo's 'Temple of Healing'). In reality armies of this period did take priests and medical personnel along on campaign, so their use here in our game isn't toooo far fetched.