We are interested in susceptibility to infection which is likely caused by a complex interplay between host and parasite. We study host-parasite interactions between the obligate intracellular eukaryotic parasite Toxoplasma gondii and its hosts. Toxoplasma can infect virtually any cell and causes life-long chronic infections (toxoplasmosis) in most warm-blooded animals. It is estimated that 30% of humans are infected with Toxoplasma. Although most infected individuals are asymptomatic, Toxoplasma can cause severe disease in immunosuppressed individuals and fetuses and also causes ocular disease in otherwise healthy people. Toxoplasma virulence differs, often quite dramatically, depending on the infecting strain and the host. The focus of the Saeij laboratory over the last years has been to identify genes of Toxoplasma that modulate the host cell and/or determine virulence, host genes and pathways that determine resistance/susceptibility, and to characterize their specific interactions. To achieve this we use a combination of genetics, genomics, biochemistry, microscopy, immunology and computational tools.