Background
Microcephaly is broadly defined as a small head size, typically greater than two standard deviations below normal, as measured via occipital frontal circumference, a method by which a tape measure is used to assess the circumference encompassing the occiput and frontal bones (forehead). It is a clinical sign and not a disease.
Microcephaly has recently gained notoriety in the popular press because of its association with the Zika virus, one of many viruses that may lead to microcephaly by first damaging the brain.
However, causality has not definitely been proven.