The CDC issued a warning to pregnant women to avoid unnecessary travel to the area of Miami.
Fourteen cases of Zika virus infections have been reported in northern Miami since Friday, leading to fears of a growing outbreak of the disease, according to government officials. The recent developments have called into question why officials didn't issued an advisory sooner.
"What we realized over the weekend was that the mosquito counts weren't coming down as we'd like so we haven't achieved good mosquito control in that area," Frieden admitted. "We now have our top experts on the ground working with the top folks in Florida to figure out what more can be done to knock down this very difficult to control mosquito."
Frieden listed several reasons, including that the mosquitoes were possibly resistant to insecticides, that not enough breeding sites were eliminated and that overall that this type of mosquito is "really tough" to control.
Other countries, including the United Kingdom, have already issued warnings for their pregnant citizens about traveling to the affected area in Florida.