There was something about the prospect of a struggle of wits with a man—with any man—that put her on her mettle and, after months of battling against countless discouragements, the knowledge that she was at last facing a definite adversary, one whom she might unhorse by her own efforts, gave her a buoyant sensation.
There are no more uses of "adversary" in the book.
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Then, again, our military training is in many respects superior to that of our adversaries.
Thucydides -- Pericles’s Funeral Oration
Finally, it was rumored, there would be complete information-sharing among all of these previously disconnected and even adversarial entities, and when they were coordinated, and once all the health data they’d collected was shared, most of this made possible through the Circle and more important, TruYou, viruses could be stopped at their sources, diseases would be tracked to their roots.