Different hosts may not contribute equally to parasite transmission. For instance, some individuals or species may be more heavily infected than others, an observation that led to the `20/80' rule, stating that in many cases 20% of the individuals are expected to be responsible for 80% of the transmission. However, studies on heterogeneity in transmission have primarily focused on intrinsic factors of transmission, such as susceptibility and infectivity, while the impact of extrinsic factors, such as connectivity network among individuals or species of the host community and the role of various host movements, has been relatively neglected. This thesis investigates the role of extrinsic transmission heterogeneities on the spread of infectious disease in multi-host systems, using tick-seabird-microparasite systems as empirical models for theoretical investigations. Four main causes of heterogeneity in transmission of vector-borne diseases were considered : (i) heterogeneity in vector abundance, distribution, and estimation thereof (ii) heterogeneity in contact among species in a multi-host community, (iii) heterogeneity in infection spread caused by different types of host movement behaviors (notably the potential role of ‘prospecting’ by host individuals among groups of hosts), and (iv) heterogeneity in dispersal ability and transmission competence among vectors with different life-history traits (stage-dependent dispersal). Our findings point to a number of potentially important factors that need to be considered more explicitly in empirical studies and which could have implications for evaluating the efficacy of disease management programs. First, we highlight the need to accurately estimate ectoparasite abundances with hierarchical modeling approaches that can take into account both heterogeneity in their detection probability and their aggregated distribution among hosts. Next, using network theory to examine the impact of community context on disease transmission and maintenance, we found that network structure (modularity, nestedness) and node-based measures (e.g., centrality) both shape the emergence of ‘super-spreader’ species (i.e. species that contribute disproportionally to disease transmission) and keystone species (i.e. species that contribute disproportionally to disease maintenance) in multi-host, multi-vector pathogens communities. Finally, we explored the contribution of host behavior and vector life-history traits to the spread of infectious agents. By reviewing the recent literature, we e highlight the fact that prospecting, relative to various other types of host movements, may be of key importance to disease transmission among groups of hosts, notably in social species. By developing a simple theoretical model, we also stress how vector life history characteristics (notably the length of bloodmeals) and demographic constraints (Allee effects) affect their colonization potential. Soft ticks, which take a single long bloodmeal at only the larval stage, are predicted to have much lower colonization rates than hard ticks, which take a single, long bloodmeal at every life stage. These stage-dependent dispersal discrepancies may have direct consequences for the genetic structure of their populations and the spread of vector-borne infectious agents. Overall, these findings highlight the importance of studying the causes and consequences of transmission heterogeneity in multi-host, multi-vector systems.  A series of potentially important sources of heterogeneity in parasite transmission are outlined together with perspectives of empirical and theoretical work to further explore their implications, which can be important for a better understanding of the ecology and evolution of host-parasite interactions, but also for disease management purposes.