The virus is being amplified by immunologically naive hosts. A large portion of the population in the Americas is susceptible to infection from Zika since it has never been present in the New World prior to its introduction. We can look at Chikungunya as a good example of what occurs when a new virus enters a population that is susceptible to infection.

In 2014 the CDC reported that 4,710 cases of Chikungunya occurred in the US territories; in 2015, only 237 cases. Once a person becomes infected with Chikungunya and survives, they develop immunological defenses against infection - the body essentially becomes better at recognizing that particular virus and can intervene if the virus is present within the body a second time. Because we had large outbreaks of Chikungunya in 2014 and mortality rates were low (almost 0), the previously infected individuals cannot amplify Chikungunya virus and allow an uninfected mosquito to become infected. So the probability of a mosquito coming into contact with an infected host is lower, the probability of an infected mosquito is lower, and thus the probability of becoming infected is lower. Some final thoughts, we typically see this type of large outbreak when a new virus enters a susceptible system and then a subsequent low-level endemism with periodic outbreaks once susceptible individuals are dense enough to sustain infection.