Researchers have found Zika virus in the brains of several infants with microcephaly, bolstering suspicions that the virus causes the birth defect. In a paper published yesterday inThe New England Journal of Medicine, researchers report on the case of a Slovenian woman who was living in Brazil when she became pregnant in February 2015 but returned to Slovenia late in the pregnancy. She had had symptoms of Zika in the 13th week of her pregnancy. After ultrasound examinations at 29 and 32 weeks showed evidence of severe brain malformations, the woman decided to have an abortion. The woman’s doctors in Slovenia found Zika virus—indicated with an arrow in this photo—in the baby’s brain and signs of malformation in the placenta. In another paper, published yesterday in Morbidity and Mortality Weekly Report, researchers from Brazil and the United States report that Zika virus was present in brain tissue from two newborns in the Brazilian state of Rio Grande do Norte who were born with microcephaly and died a few hours after birth. The babies’ mothers had symptoms of Zika early in their pregnancies, but they were healthy when the babies were born. (The mothers were not tested for the virus.) The researchers also found the virus in placental and fetal tissues from two women who miscarried early in their pregnancies after suffering symptoms of Zika infection. Although the findings strengthen the evidence that Zika virus is involved in the increase in microcephaly cases seen in Brazil, it's still unclear what fraction of infected mothers pass the virus to their unborn babies and what percentage of infected babies suffer from developmental defects. Those questions will only be answered by longer term studies, which are underway in several countries.