We have studied the factors influencing the virulence of diseases, i.e. the severity of their symptoms, to try to understand how and why pathogens adapt to become lethal or on the contrary benign. By using again the Microbotryum fungi as biological models, we carried out one of the first theoretical studies validating some important theoretical expectations: 1) multiple infections (i.e. the presence of several genotypes of a given pathogen in the same host) increase virulence, while 2) genetic relatedness between pathogens within a given host decreases virulence by increasing kin cooperation; 3) castrating parasites are selected to increase the survival of their host, to be able to be transmitted longer.

Overall, these studies contributed to a better understanding of 1) how new diseases can emerge in natural ecosystems and agrosystems, 2) how pathogenic fungi adapt to new hosts or global changes, 3) what are the factors controlling disease virulence. These questions are not only academic exercises; they have important applied consequences, in agriculture and medicine, but also for the maintenance of the biodiversity.