Introduced species escape many pathogens and other enemies, raising three questions. How quickly do introduced hosts accumulate pathogen species? What factors control pathogen species richness? Are these factors the same in the hosts’ native and introduced ranges? We analysed fungal and viral pathogen species richness on 124 plant species in both their native European range and introduced North American range. Hosts introduced 400 years ago supported six times more pathogens than those introduced 40 years ago. In hosts’ native range, pathogen richness was greater on hosts occurring in more habitat types, with a history of agricultural use and adapted to greater resource supplies. In hosts’ introduced range, pathogen richness was correlated with host geographic range size, agricultural use and time since introduction, but not any measured biological traits. Introduced species have accumulated pathogens at rates that are slow relative to most ecological processes, and contingent on geographic and historic circumstance.