Sentence:  In his first book since the 2008 essay collection "Natural Acts: A Sidelong View of Science and Nature", David Quammen looks at the natural world from yet another angle: the search for the next human pandemic, what epidemiologists call “the next big one.” His <WSD>quest</WSD> leads him around the world to study a variety of suspect zoonoses—animal-hosted pathogens that infect humans. 
Meaning of the word marked with <WSD> tags:  A journey or effort in pursuit of a goal (often lengthy, ambitious, or fervent); a mission.