The numerous essential services provided by biodiversity to society render the current sixth mass extinction event a major societal challenge, which is exemplified by a global and massive loss of amphibian diversity. The fungal amphibian disease chytridiomycosis, which is caused by two chytrid fungi, Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bs), is regarded the greatest infectious threat ever recorded to biodiversity. The recent emergence of both fungi in northwestern Europe with unknown consequences for biodiversity urges to replace the classical concept of individual infectious threats to biodiversity by a more dynamic multifactorial concept taking into account the costs incurred by multiple pathogen exposure on animal and community fitness. Here, we will first determine whether co-occurrence of Bs and Bd results in additive, synergistic or antagonistic effects on the amphibian host by quantifying fungal dynamics, disease severity and host range. We will then study the mechanisms underpinning the observed effects by comparing host and pathogen responses, fungal niche occupancy and amphibian skin defenses (microbiome, antifungal skin compounds, skin integrity) between animals infected with a single fungus and those infected with both fungi. Finally, we will assess the impact of co-occurrence of Bs and Bd on an amphibian community, consisting of multiple amphibian species with differential susceptibility to the fungal infections, using a mesocosm set-up.