Disease ecologists study host-parasite/pathogen interactions within the context of their environment and evolution. This is important because outbreaks of infectious diseases are frequently caused by changes to the ecology of the host, the parasite/pathogen, or the environment.

Parasites likely outnumber free-living species and play important roles in communities and ecosystems. These include regulating host populations, altering species interactions, and adding to food web complexity. Changes in host-parasite-environment interactions can have many possible consequences.