The World Health Organization (WHO) is looking into the specific reasons for the increased incidence of Zika virus infection.3 At the moment, it appears that the most likely reason is that the virus is well adapted to living and reproducing in mosquitoes, and is capable of reproducing in the human body—which allows it to remain in human blood for a longer period of time and multiply to greater numbers. Since the virus is spread if an infected person or monkey is stung by a mosquito, the virus has adapted to these three hosts. If the virus could replicate better in the three hosts, the amount of virus in mosquitoes, monkeys and humans will increase, thus making virus transmission more effective.