The World Health Organization and its director general, Dr. Margaret Chan, were right to declare the Zika virus an international public health emergency, even if its suspected link to severe birth defects has not been proved. The mosquito-borne disease is a serious threat: It is usually so mild as to be undetectable in adults, yet as it has exploded across South and Central America it has been followed by a surge in babies born with underdeveloped heads, a condition called microcephaly.
The emergency designation will galvanize coordinated international monitoring and action of the sort that was tragically missing in the first months of the Ebola pandemic.