Biodiversity is declining dramatically. Meanwhile, disease risk of infectious disease in human, wildlife and livestock is undergoing an increase. Several studies showed that a change in biodiversity can impact disease risk. The objective of this thesis is to advance the understanding of the relationships between disease risk and biodiversity. My results give new insights into these relationships. My findings suggest that the commonly used species richness is not a good indicator of disease risk, and that assemblage composition (i.e., abundance and relative abundance of species) and structure of wildlife assemblages (i.e., functional diversity and phylogenetic diversity) are more important than species richness in affecting disease risk. My results highlight the role of phylogenetic relatedness in the relationships between disease risk and biodiversity, and showed that if the species in an assemblage are closely related, especially among competent hosts species, the assemblage is expected to have a higher disease risk. My study suggests that future studies should look beyond species richness when studying relationships between biodiversity and disease risk.