So we already know that in 1.4 villagers will be able to 'infect' villagers. But what can be used to reverse this effect? In today's snapshot it looks like the community found out: a potion and a golden apple. The forum post in the source link explains that the process involves, "using a Weakness Splash Potion followed by a golden apple on a villager zombie will revert them back to a normal villager! They emit red squiggles and, after about 5-10 minutes, will transform back into a villager." Now that golden apples need golden blocks, it's kind of an expensive way to get your villager buddies back from the dead.
Source: Minecraft Forum