New study disproves suspicion that dengue raises risk of zika-related microcephaly A pregnant woman infected by zika virus does not face a greater risk of giving birth to a baby with microcephaly if she has previously been exposed to dengue virus, according to a Brazilian study that compared data for pregnant women in Rio de Janeiro and Manaus. A zika epidemic broke out in Brazil in 2015-16 in areas where dengue is endemic. Both viruses are transmitted by the mosquito Aedes aegypti. Some of the states affected by the zika epidemic reported a rise in cases of microcephaly, a rare neurological disorder in which the baby's brain fails to develop completely. Others saw no such rise.