How about making weakened villagers study the cactus? It seems to me that the healers don't get sick as often, esp masters (maybe it's just me). So when one of them gets sick for too long or is overworked, I just tell them to become a healer and, although I haven't kept records or anything, they've seemed to either get better or (if they're elderly) stay alive much longer.

And I'm not so sure on the weak + breed = healthy thing. Whenever I tried it, she only got weaker, luckily the babies have been able to separate before their mothers died.
-spaceman