In this special issue the focus is on the factors and (molecular) mechanisms that determine
the transmission eciency of a variety of insect pathogens in a number of insect hosts. In this editorial,
we summarize the main findings of the twelve papers in this special issue and conclude that much
more needs to be learned for an in-depth understanding of pathogen transmission in field and cultured
insect populations. Analyses of mutual interactions between pathogens or between endosymbionts
and pathogens, aspects rather under-represented in the scientific literature, are described in a number
of contributions to this special issue.