You may be mistaken. It does matter who you choose to volunteer for. Try to stick with established organizations with solid community recognition and support. This is not a good country for a foreigner to venture into the countryside or the barrios to do "good works" in the absence of other volunteers and with people who could have an alternative objective.
There are lots of legitimate opportunities to work with animals, children and the poor. Keep your eyes open, consider your safety and don't go off somewhere without telling someone where you are going, with whom and when you expect to return.
Thanks for wanting to help, but your safety and well being should come first. If you plan to be rummaging around squalor, best to have all your vaccinations up-to-date.