This is disturbing and sad. A tropical disease that is spread by mosquitoes and that seems to be moving north—and may be approaching the US border—is being linked in health-agency reports to a disabling birth defect.

The disease is Zika virus, and the birth defect is microcephaly: smaller than normal heads in infants, due to the brain having stopped growing at some point in pregnancy. According to the World Health Organization, Brazil has reported 739 cases of microcephaly among newborns in nine states in the “shoulder” of Brazil, the northeastern corner that protrudes into the Atlantic. Brazil’s Ministry of Health says there are more: 1,248 suspected cases, in 14 states. Seven children have died, and five others’ deaths are being investigated, according