Transmission heterogeneity emerges from uncoupled parasite shedding and spreading

Disease transmission is highly heterogeneous, yet hosts are often classified as high- or low-risk transmitters based on single metrics such as parasite load or shedding. Here, we test whether transmission-associated processes, which are commonly assumed to covary, remain coupled across host genotypes and infection routes. Using interactions between Caenorhabditis nematodes and a wild bacterial parasite, we decompose transmission into sequential steps spanning the infection cycle, including host route sensitivity, parasite shedding, infection-induced dispersal, and downstream infection outcomes in recipient hosts. These components varied largely independently across host genotypes and ecological contexts, leading to repeated reordering of host transmission profiles across stages. Parasite shedding alone did not consistently predict downstream infection outcomes. Parasites from different hosts generated divergent infection burdens and virulence in standardized recipient hosts, even at comparable exposure doses. This result suggests that host background may influence not only parasite quantity but also parasite performance after transmission. Ecological context reshaped virulence by altering the consequences of environmentally acquired versus host-acquired infection. Together, our results show that transmission heterogeneity emerges from distinct ecological bottlenecks across the infection cycle. By separating transmission quantity from transmission quality, we show why more parasites do not necessarily mean more transmission.