Sentence:  Bats host many high-profile viruses that can infect humans, including severe acute respiratory syndrome and Ebola. A recent study explored the ecological <WSD>variables</WSD> that may contribute to bats' propensity to harbor such zoonotic diseases by comparing them with another order of common reservoir hosts: rodents. 
Meaning of the word marked with <WSD> tags:  Something whose value may be dictated or discovered.