The fight against the Zika outbreak in Puerto Rico will be difficult, according to Thomas Frieden of the U.S. Centers for Disease Control and Prevention (CDC).

The CDC director reiterated during his visit to the island the importance of protecting women, particularly those who are pregnant, from the mosquito-born virus, which has been linked to two health conditions. There has been a spike in the number of microcephaly cases in Brazil, where the virus has been the most widespread.

The Brazilian Health Ministry reported late last month that the country is investigating a total of 5,640 reported cases of microcephaly. So far, there have been 583 confirmed cases of the defect.

Microcephaly is a condition that occurs when an infant is born with an abnormally small head and an underdeveloped brain. While experts have not found a direct link between the virus and the defect, the evidence compiled so far suggest that the link is strong.

"Until a few months ago, no one had any idea that Zika could cause birth defects," Frieden said in a news briefing in Puerto Rico.

The purpose of Frieden's three-day trip was to get a first-hand assessment of the current situation and discuss the important measures that need to be taken. 

"Dr. Frieden is traveling to the island to emphasize the need for preparedness and prevention to reduce the public health impact of the Zika outbreak in Puerto Rico, with a focus on reducing the risk of Zika virus to pregnant women. During his visit, Dr. Frieden will meet with key public health officials and local leaders to assess how CDC and other U.S. government partners can best support Puerto Rico's response to the outbreak," the CDC wrote in a news release.

Freiden, who described Zika as a "tremendous challenge and crisis," has been working with CDC health officials and members from the Puerto Rican government. He said during his stop at the CDC's Dengue Branch location in San Juan that testing the effectiveness of insecticides is a vital part of the Zika response.

The CDC director currently recommends everyone to use insect repellent every day and eliminate standing water in order to reduce the risk of an infection, as standing water can become breeding grounds for the Aedes aegypti mosquito. He added that residents should add window screens and air conditioning units if they have the means to.

CDC researchers have also been actively tracking the outbreak in Puerto Rico and looking for the best ways to prevent infections. The U.S. Department of Health and Human Services has been arranging shipments of clean blood to Puerto Rico.

Freiden and the CDC do not expect widespread transmission to occur in the U.S but noted that clusters are expected.