Perhaps their immune systems don't know how to fight it, they have a genetic susceptibility or prior infections with dengue made Zika worse (SN: 4/29/17, p. 14). (sciencenews.org)
There have been confirmed cases in 30 States in the U.S. Dogs are highly susceptible to the disease since they have no immunity from prior infections. (dog-health-guide.org)
As they describe, their «ability to forecast vaccination impact is primarily limited by the current uncertainty regarding whether and how vaccine efficacy wanes over time and how vaccine efficacy is affected by prior infection.» (sciencedaily.com)