If you have a large medkit on hand, let a single Walker slap you once, get burned for a short second or drown yourself in a pool for a second - then use it right away. You'll need to have lost less than 1/2 of a health bar for this to work.

Florencio at the Warehouse in Moresby sells these if you can't find one. At the beginning of Chapter 18 you will always get one of these if you can't find it anywhere else though. Load that in a NG+ if necessary.