Animal or plant species may be hosts for a range of pathogens, and most host populations encounter a large number of different pathogen species [8] . For significant host species, there is abundant evidence of differences in the infection frequency of the various pathogen species present in an ecosystem. The distribution of pathogens over their hosts, and the distribution of different pathogens within a host species, will affect the frequency of multiple infection of an individual host by different pathogens. Multiple infection may have important consequences for the infected hosts, for the pathogens, and for host-pathogen coevolution [8, 9] . In the host, frequent co-infections may lead to heterozygote superiority against multiple pathogens and contribute to the persistence in host populations of alleles conferring susceptibility to disease [10] . In multiple infected hosts, pathogens can cooperate or can compete for host resources, which will affect each other's fitness. Hence, multiple infections will be a factor in pathogen evolution. Theoretical analyses predict that the withinhost dynamics of microparasites in multiple infected hosts may have important consequences in the evolution of their virulence [11] [12] [13] [14] , and there is evidence that multiple infection may result in either increased or reduced virulence [e.g., [15] [16] [17] . Multiple infection of a host may also directly affect the genetic diversity of the pathogen population, as co-infection is a prerequisite for genetic exchange between different pathogen species or strains. Also, infection by one pathogen may result in an increased host susceptibility to a second pathogen, a common phenomenon named facilitation or predisposition by animal and plant pathologists, respectively [8, 18] .