I am aware that birds are also a common vector, but bats seem to be the greater risk.
> In vitro work has suggested that molecular adaptations that support the physiology of flight, a trait unique to bats among mammals, may allow bats to tolerate rapidly replicating viruses that express heightened virulence upon emergence in less tolerant hosts such as humans — thus offering a possible explanation for bat virus virulence. Bats and birds share a suite of convergent flight adaptations—both taxa are remarkably long-lived for their body size and appear to circumvent metabolic constraints on longevity through cellular pathways evolved to mitigate oxidative stress induced by flight.
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I recommend the book "Spillover" about animal-borne diseases which looks at it in detail.