The Zika virus is primarily spread by mosquitoes. While most people who catch it experience mild or no symptoms, there are concerns that the fetus of infected pregnant women may be at risk for a rare birth defect that causes brain damage and an abnormally small head. President Obama is asking Congress for a historic $1.8 billion to combat the Zika virus — though he previously tried to cut funding to the same agency now responsible for tackling the virus sweeping South and Central America and the Caribbean. In an announcement Monday, the White House said the money would expand mosquito control programs, speed development of a vaccine, develop diagnostic tests, and improve support for low-income pregnant women. “While we have not yet seen transmission of the Zika virus by mosquitoes within the continental United States, Puerto Rico and other U.S. territories in warmer areas with Aedes Aegyptus mosquito populations are already seeing active transmission,” the White House warned in a statement.