It is very difficult for third world country researchers to publish papers due to hefty page charges. The reason is the disparity in value of currency. For example, one US dollar is equal to 62 Indian rupees.
Namely PLOS neglected tropical diseases, they have support for researchers from developing countries to publish, either by not charging at all or just a "small" fee (there are two lists on the website). There's more info here http://www.plosntds.org/static/developing
Also check PLOS Pathogens, although could not find similar info, but it is a question of asking them.
Several Elsevier and Springer journals are free of charge. If your topic is one with a more epidemiological background about tropical diseases, I recommend submitting your manuscript to Acta Tropica.