Where in the U.S. Have Zika Cases Been Reported?

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Nearly 20 people have been diagnosed with the Zika virus in the U.S., in addition to 19 cases reported in Puerto Rico. A model of a mosquito is shown during a campaign in the health ministry in Lima, Peru, to raise awareness about the Zika virus, January 27. Mariana Bazo/Reuters

A teenage girl in southern California is the only known person to have become infected with the Zika virus in the state, the Los Angeles County Health Department said on Tuesday.

The girl, who has since recovered, traveled to El Salvador in late November 2015, the department said in a statement. The Central American nation is one of 24 countries and territories in the Americas, the Pacific Islands and Africa with active transmission of the virus, according to the Centers for Disease Control and Prevention (CDC).

The girl was tested for the Zika virus at the end of December and Los Angeles County recently received the results, says Dr. Benjamin Schwartz, deputy director of the acute communicable disease control program at the L.A. County Department of Public Health. She was not pregnant and had the typical symptoms of joint and muscle aches, a rash and conjunctivitis, a feature of some other infectious diseases, Schwartz tells Newsweek. No known transmission of the virus occurred in Los Angeles County and "remains unlikely," although there is ongoing surveillance to ensure no one else became infected, the county said.

Cases of Zika have been reported elsewhere in the U.S. On Tuesday, Virginia officials said a state resident tested positive for the Zika virus. The same day, Arkansas authorities also confirmed a positive case. Both people recently traveled abroad to countries with active transmission. Hawaii saw the first case of brain damage related to Zika earlier this month, while cases have also been reported in Florida, Illinois, New Jersey and Texas.

Zika, a similar virus to Aedes aegypti mosquito-borne dengue, chikungunya and yellow fever, has erupted into a public health crisis in Brazil, where it was first reported last May. Around 4,000 babies in Brazil have been diagnosed with microcephaly, a condition that stunts brain growth and sees children born with abnormally small skulls. There's evidence that links the Zika virus to the increase in microcephaly cases, and pregnant women and those trying to conceive have been urged not to travel to countries where the disease has been reported.

In Puerto Rico, 18 cases of the disease have been confirmed. On Friday, the CDC released a report that said international air travel means the virus is likely to spread to the U.S.

"There will definitely be more cases identified in travelers who go to areas where the infection and where outbreaks are ongoing." says Schwartz. He adds that people who have traveled to those areas must also take precautions when they return to the U.S.: if they're infected with Zika and bitten by a mosquito in the U.S., that mosquito can then bite and infect someone else.

On Friday, the CDC expanded its travel alert to eight additional countries and territories where the disease poses a risk—Barbados, Ecuador, Guadeloupe, Cape Verde, Samoa and the island of Saint Martin. There is no vaccine or treatment for the virus. The most common symptoms are fever, rash, joint pain and conjunctivitis.