Make New Friends and Keep the Old? Parasite Coinfection and Comorbidity in Homo sapiens

Across species, the fi tness costs of parasitic infection have been a major force shaping host adaptations to avoid infection (Hart 2009 ; Schmid-Hempel 2003 ; Sheldon and Verhulst 1996 ), diminish the cost of infection (Minchella 1985 ; Råberg et al. 2009 ), and even advertise resistance to infection to possible mates (Hamilton and Zuk 1982 ; Moller 1990 ). These adaptations, in turn, have shaped selection on parasite transmission and virulence, leading to coevolved host–parasite systems. Host–parasite interactions are further shaped by local environments and proximate host factors that infl uence transmission risk and infectious outcomes, including age, sex, and nutritional and immune status (Anderson and May 1981 ; Anderson 1991 ; Quinnell et al. 1995 ; Schad and Anderson 1985 ; Woolhouse 1992 ). Host–parasite interactions are often observed and modeled as hosts interacting with a single parasite species. Yet as is increasingly observed in animal populations, including humans, coinfection with two or more species (alternately termed “multiplespecies” or “polyparasitic” infection) may be the rule in nature (Howard et al. 2001 ; Booth et al. 1998 ; Bordes and Morand 2009 ; Pullan and Brooker 2008 ). Coinfecting species may include any number of “typical parasites” (e.g., helminths, fl ukes, tapeworms) and/or pathogens (e.g., bacteria, viruses, protozoa), each with different associated exposure risks, infectious sites, reproductive strategies, virulence, Make New Friends and Keep the Old? Parasite Coinfection and Comorbidity in Homo sapiens