United States officials are expecting more than 4 million cases of the Zika virus to occur in just the Western hemisphere, according to a new government oversight report.

The U.S. Centers for Disease Control and Prevention recorded “107 cases of continental U.S. travel-associated Zika virus” as of Feb. 24, according to the report, which notes that several U.S. states have mosquito populations capable of transmitting the virus, the Washington Free Beacon reports.

Government officials warn that the virus is likely to spread throughout the Americas, with intelligence officials predicting more than 4 million cases in the Western hemisphere alone.

As the Zika virus outbreak reaches “epidemic levels,” the CDC is warning that aspects of the disease have yet to be understood.

“The virus has continued to spread to the Americas, with the outbreak in Brazil that began in May 2015 and is ongoing,” according to the report. “Zika has spread to Mexico, Central America, the Caribbean, and South America, where the outbreak has reached epidemic levels.”