Kelli Fabick of Ellisville, Mo., held her mother's Australian silky terrier, Zoey, as veterinarian Sarah Hormuth gave the dog a flu shot last April. The dog flu strain H3N2 is now circulating in more than two dozen U.S. states. St. Louis Post-Dispatch/TNS via Getty Images hide caption

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Rod-shaped specimens of Yersinia pestis, the bacterial cause of plague, find a happy home here in the foregut of a flea. Fleas can transmit the infection to animals and people, who can get pneumonic plague and transmit the infection through a cough or kiss. Science Source hide caption

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