I try to understand why multipartite viruses evolved and how they function. These viruses are common (~20% of known virus genera), yet, perhaps because they almost exclusively infect plants and fungi, their genomic architecture and ensuing way of life is still poorly understood: their genome consists of several segments, each encapsidated in a different viral particle (yes, you read correctly). They must thus somehow ensure their genomic integrity, while no obvious advantage specific to this genomic architecture has yet been identified. Our work has produced some surprising results: (i) the different segments occur in host-species-specific distributions, some being frequent while some are rare, which change upon host switch; while this adds to the potential cost of genomic integrity (rare segments are easier to lose) it allows these viruses to rapidly modify their gene expression; (ii) the different segments do not need to all be present in the same host cell, they may traffic their function (protein or mRNA) among cells. This greatly reduces the within-host genome-integrity issue. But other important issues remain; we’re working on them. I am also interested in why parasites harm their hosts, why they specialize (or not), host/parasite local adaptation, the evolution of host/parasite life-history traits, the evolution of reproductive mode… ORCID: 0000-0003-1929-0848