Abstract
A major challenge in disease ecology is to understand how co‐infecting parasite species interact. We manipulate in vivo resources and immunity to explain interactions between two rodent malaria parasites, Plasmodium chabaudi and P. yoelii . These species have analogous resource‐use strategies to the human parasites Plasmodium falciparum and P. vivax : P. chabaudi and P. falciparum infect red blood cells (RBC) of all ages ( RBC generalist); P. yoelii and P. vivax preferentially infect young RBC s ( RBC specialist). We find that: (1) recent infection with the RBC generalist facilitates the RBC specialist ( P. yoelii density is enhanced ~10 fold). This occurs because the RBC generalist increases availability of the RBC specialist's preferred resource; (2) co‐infections with the RBC generalist and RBC specialist are highly virulent; (3) and the presence of an RBC generalist in a host population can increase the prevalence of an RBC specialist. Thus, we show that resources shape how parasite species interact and have epidemiological consequences.