Pike, the malingerer, leaped upon the crippled animal, breaking its neck with a quick flash of teeth and a jerk, Buck got a frothing adversary by the throat, and was sprayed with blood when his teeth sank through the jugular.
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.