Sentence:  To pull this off, the authority required three things: the ability to identify cases in a timely fashion, and so determine the infection's direction of travel; an understanding of how the disease spread (by water? air? insect vector?), and hence the measures that were likely to block it; and some means of ensuring compliance with those measures. When all three of these ingredients – which we'll describe in more detail in the following sections – were in place, containment could be extremely effective, but a <WSD>hat-trick</WSD> was rare. 
Meaning of the word marked with <WSD> tags:  Three incidents or achievements that occur together.