The environment antagonises the protagonists through regular floods and outbreaks of toxic bacteria.
Although, if you wanted to look more deeply, you might notice that 'environment' in this instance doesn't refer to nature, it refers to a human-made environment, so you could say that the real antagonist in this snippet is a wider social system that deprives indigenous people of native land, confines them to a reserve, and then fails to invest in its maintenance and care.