Macmillan Learning Stacked Until recently, the Zika virus was a rare and mild public health concern and little was known about it. In early 2015, Brazil reported an epidemic of Zika viral infection. Reports soon followed of unusual numbers of neurological disorders such as microcephaly in newborns, a condition characterized by a very small head and poor brain growth. Explain why we cannot conclude from this information alone that Zika infections cause microcephaly in newborns. It is not biologically plausible for the Zika virus to cause brain issues for newborns. One needs to conifirm that microcephaly and brain defects occur before the Zika virus infection. There may be a lurking variable that is causing an increase in both Zika infections and microcephaly in newborns. Correlation and regression lines describe only linear relationships, so no conclusions can be drawn.