First, we propose indices for the observed patterns of host infection by different pathogens, and the observed patterns of coinfection, and tests to analyse if they conform to the null hypothesis of randomness or reflect associations. Second, we apply these tests to data on the prevalence of five insect-borne virus species in wild plant species within an agroecosystem in Central Spain. Results of these analyses uncover patterns that, if general, would be highly relevant to understand the ecology and evolution of pathogens. [19] . Except for TSWV, which has a single-stranded RNA genome of negative and ambisense polarity, all other viruses have single-stranded RNA genomes of messenger polarity. AMV, CMV and WMV are transmitted by aphids in a non-persistent manner, i.e. the virus is retained in the distal structures of the aphid mouth parts for short period of time. BWYV is transmitted in a circulative, non-propagative manner, i.e., the virus penetrates through the gut wall into the haemocoel of the insect vector, and circulates with the haemolymph to reach the salivary glands, from where it is inoculated into new plants. TSWV follows a similar path within the thrips body, but infects and multiplies in the insect cells [19] . All five viruses cause important diseases in vegetable crops worldwide, including the studied region in Central Spain, but infection in the analysed wild hosts was asymptomatic. Table 1 shows the number of samples analysed and the number of infected plants by each of these five virus species, in single or multiple infection, in the 21 most frequently found plant species in three monitored habitats (see Methods) for the analysed period.