Don't kill all the frogs! Keep 2-3 around so villagers will get sick. Your healer-in-training can then heal them and advance faster. Villagers don't get sick very quickly so you won't be having an epidemic or anything, but it helps. You can have an adult catch a frog. They will get sick about 1/2 the times you try this, but then frog is gone and you have to wait for a new one. Otherwise, just let a few frogs roam the island and a few villagers eventually get sick.