In late 2015, health officials in Brazil recognized a marked increase in the number of infants born with microcephaly (from Greek, meaning small head), a birth defect that may accompany significant, permanent brain damage. Although not conclusive, the increase in microcephaly is suspected to be related to the emergence of Zika virus infections in Brazil early in 2015. Zika virus is related to the viruses that cause yellow fever, dengue, West Nile, and Japanese encephalitis viruses. Historically, Zika virus was found in Africa. Since 2007, Zika transmission has also occurred in Southeast Asia and the Western Pacific. In the Americas, transmission was first identified in Brazil in May 2015. As of 29 January 2016, the Pan American Health Organization (PAHO) had received reports of local transmission in 25 countries and territories in the Americas, including Puerto Rico and the U.S. Virgin Islands. This book discusses scientific and technical aspects of Zika virus infection, including modes of transmission, symptoms, diagnosis and treatment, and prevention. Policy concerns and official actions are evolving and will also be discussed.