Figure 1

Observations of co-infection prevalence can misrepresent between-parasite species interactions. In the above example, exposure to antagonistic parasites has resulted in no infection, or mono-specific infection, in most hosts. 5/10 people are infected with Parasite 1 (white) and 5/10 people are infected with Parasite 2 (black). More hosts are co-infected (4/10) than might be expected from random chance (5/10 x 5/10 = 2.5/10). This would typically lead to the incorrect assertion that the parasites exhibit synergistic interactions. Only by knowing both the patterns of infection status and exposure to contaminated environment can inferences be made of within-host between-parasite species interactions.