More than 20,000 people, including at least 2,116 pregnant women, have been infected with the Zika virus in the South American country of Colombia.

Colombia's National Health Institute reported on Saturday that it has recorded 20,297 cases of the infection, which makes it the second most affected country in the region after Brazil.

The World Health Organization warned this week that the virus is "spreading explosively" in the Americas, with three to four million cases expected this year.

Although the mosquito-borne virus' symptoms are relatively mild, it is believed to be linked to a surge in cases of microcephaly, a condition in which a baby is born with an abnormally small head and brain.

Microcephaly is an untreatable condition that can cause permanent damage to the child's motor and cognitive development.

Brazil has reported as many as 1.5 million cases of Zika infection. Since the outbreak was detected there last year, 3,718 cases of microcephaly have been reported, compared to an average 163 cases a year before that.

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The National Health Institute said that 1,050 of Colombia's Zika infection cases were confirmed by laboratory tests, 17,115 by clinical exams, and 2,132 were suspected cases.

Women have been the most affected in Colombia, accounting for 63.6 percent of the cases.

The government expects more than 600,000 people to become infected with the Zika virus in Colombia this year, and projects some 500 cases of microcephaly.

On Tuesday, Colombian authorities ordered hospitals in lower-lying areas to prepare for the spread of the disease, which is carried by the Aedes aegypti, or yellow fever mosquito.

It also recommended that couples delay attempts to become pregnant for six to eight months.