Increased virulence due to multiple infection in leads to limited growth in 1 of 2 co-infecting microsporidian parasites.

Recent outbreaks of various infectious diseases have highlighted the ever-present need to understand the drivers of the outbreak and spread of disease. Although much of the research investigating diseases focuses on single infections, natural systems are dominated by multiple infections. These infections may occur simultaneously, but are often acquired sequentially, which may alter the outcome of infection. Using waterfleas () as a model organism, we examined the outcome of sequential and simultaneous multiple infections with 2 microsporidian parasites ( and ) in a fully factorial design with 9 treatments and 30 replicates. We found no differences between simultaneous and sequential infections. However, fitness was impeded by multiple infection due to increased host mortality, which gave less time to grow. Host fecundity was also reduced across all treatments, but animals infected with at a younger age produced the fewest offspring. As is both horizontally and vertically transmitted, this reduction in offspring may have further reduced fitness in co-infected treatments. Our findings suggest that in natural populations where both species co-occur, may evolve to higher levels of virulence following frequent co-infection by .