With a small enough infection, an extremely healthy person could possibly fight it off. But they would have to be pretty exceptional, or weird. For example, Reed is a dhampire, he looks and acts like a human, but could not be turned because he's already infected with antimus at a very ingrained level.

And of course, Visterothi orks can't be turned either and they are completely living. They have been genetically modified to be immune to the Unlife "viruses" (see more of this in OOA). One presumes there may be other such races.

In general, if you get to a qualified professional (any appropriate mana wielder with the knowledge of how to do so) it can be cured before you turn.

As discussed earlier, it can, in general be cured after the event, if said qualified professional knows what they are doing and has the power (or permission level for priests)