Declaring an emergency is a dirty work, but someone has to do it. When facing a serious threat to global public health, even if complete evidence is lacking, someone has to take the responsibility to push the red button that activates a chain of coordinated actions (such as cooperation among states and research on vaccines). Choosing to do this, the risk of giving a false alarm is unavoidable.
The Zika virus has been recently identified in Europe after having spread in Brazil, where its first infection was confirmed in March 2015. Despite being relatively mild, this virus is now making headlines, as it could be responsible of a large increase of microcephaly among infants. A connection that pushed Brazil to declare Zika a public health emergency.
