The number of infants born with the rare brain defect known as microcephaly continues to rise in Brazil, and 3,893 new cases have been identified since health officials began investigating the causes in October. Officials say they’re convinced that the wave of new cases is linked to an outbreak of the mosquito-borne virus known as Zika, according to the Associated Press.

Infants born with microcephaly who survive for a substantial time after birth tend to have smaller heads and brains that don’t develop properly. Fetuses often miscarry or die shortly after birth if they have microcephaly.

Until recently, the concern has mostly been confined to Brazil's poor northeast, where most microcephaly cases have been concentrated. But now more and more are turning up in the urban regions of Rio de Janeiro and São Paulo.

Zika virus infections also have begun appearing in the United States, where at least three cases have been found in Florida and another two cases involving pregnant women showed up in Illinois. There are also confirmed cases in Texas and Hawaii, including the birth of one child with a birth defect, according to CBS News. All of those cases involve travel to the Caribbean, Central or South America.

Zika virus is a similar disease to Dengue fever, a potentially lethal virus found in tropical locations and carried by mosquitoes. Incidences of Dengue fever, just like Zika, have grown recently as well, to a lesser extent. Dengue Is one of the leading cause of serious illness and death for children in some Asian and Latin American countries, according to the World Health Organization.

People infected with the Zika virus often have a mild fever or skin rash that lasts two to seven days. In addition to Brazil and the United States, it has been found in Africa, other parts of the Americas, Asia and the Pacific, according to the World Health Organization.