The informant was my cousin (referred to as LG) who spent 4 years being a Fulbright scholar in Indonesia. There she was teaching school girls English and art. She told me this proverb which she had heard from one of the host families she was staying with. My cousin got very sick and had a horrible fever and while her host family was taking care of her this:

LG: “I heard this proverb when I was really sick and had a fever of 104 degrees. It was rainy season and things were flooding and it was horrible and then I got terribly sick. My host family said, ‘Alah bisa karena biasa,’ which roughly translated to ‘One gains immunity against poison when exposed to it regularly.’”
This obviously is meant to be interpreted as “what doesn’t kill you makes you stronger.” I think it is interesting they told my cousin this when she was really sick, as if perhaps they were saying she was weak for getting sick due to the flooding, because they had not fallen ill. Perhaps they felt they were immune to the sickness because they “were exposed to the poison regularly.” I believe this idea probably stems from living in poverty and for the most part, when something goes wrong there might not be a lot you can do about it, so it is a positive way to look at any negative situation. Essentially, you’re building immunity for the next time something bad happens.