The number of pregnant women in the United States and its territories who have been infected with the Zika virus has spiked dramatically in the past month because of a change in how the Centers for Disease Control and Prevention reports the data.

Based on the CDC's new reporting system, 279 women in the U.S. and Puerto Rico have been infected with Zika during their pregnancies. Some of the women were infected by sexual transmission, an alarming reminder that direct mosquito bites aren't the only way to contract the disease.

Since February, the CDC has monitored U.S. pregnant women infected with the Zika virus. For months the number of cases appeared to rise steadily and by April stood at 48 in the continental states. But that number, which the agency reported publicly, only reflected women who tested positive for the Zika virus and showed symptoms.

On Friday, the CDC announced it is now going to publicly report not only the number of pregnant women who test positive and show symptoms of the virus, but also pregnant women who display no symptoms but have shown laboratory evidence of possible exposure.

Researchers say the new system will create a better understanding of the range of problems the virus inflicts in utero.

Four out of five people who get Zika show no symptoms. Those who do develop a rash, sore joints, fever and conjunctivitis, which can last a week to 10 days. The mosquito-borne virus, however, remains in blood and semen much longer.

The CDC waited until now to report the greater number of affected women because of the rapid changes in what's known about the disease's impact on pregnancy, said Margaret Honein, chief of the CDC's birth defects branch and co-lead of the Pregnancy and Birth Defects Team with the CDC Zika Virus Response Team.

“Our goal is to track all Zika-affected pregnancies,” said Honein. “We want to be transparent about the numbers of affected women.”

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