Travelers who recently returned to California, Arkansas and Virginia from foreign countries have tested positive for the Zika virus, health officials in those states announced Tuesday.

The Centers for Disease Control and Prevention (CDC) maintains that there haven’t been any people who have contracted the mosquito-borne virus in the United States, though there have been several cases where travelers got infected overseas and brought the virus back with them.

Zika is collecting the attention of public health officials because scientists are studying a possible link between the virus and a rare condition called microcephaly, in which children are born with smaller-than-usual heads. The birth defect can also be caused by other factors, the CDC says.

Still, the CDC has issued travel notices for 22 countries or territories where Zika is currently being spread, urging pregnant women to consider postponing any planned travel to those areas and asking all would-be travelers to “practice enhanced precautions” to prevent mosquito bites.

The three cases announced Tuesday all involved foreign travel.

In a statement, the Virginia Department of Health said the infection was confirmed in “an adult resident of Virginia who recently traveled to a country where Zika virus transmission is ongoing,” but did not elaborate. The infected individual was the state’s first imported Zika case, but isn’t at risk of transmitting the virus because it isn’t currently mosquito season in Virginia.

The Arkansas Department of Health said one of the state’s residents “recently traveled out of the country and had a mild case of Zika.” Officials confirmed the diagnosis late Monday afternoon.

“Arkansas has the kind of mosquitoes that carry Zika virus, so mosquitoes here in Arkansas can become infected with the virus if they bite someone who has Zika,” Dr. Nate Smith, the Arkansas Department of Health Director and State Health Officer, said in a statement. “For this reason, people traveling to countries with Zika should avoid mosquito bites for 10 days after they return.”

The Los Angeles County Department of Public Health said an adolescent girl who traveled to El Salvador last November was the county’s lone confirmed case of Zika, but she has recovered.

“At this time, local transmission is unlikely,” the department said in a statement. “It would require an Aedes mosquito biting a Zika infected person and then biting others.”

The CDC says only about 20 percent of people who are infected with Zika become ill and develop symptoms like fever, rash and joint pain. Most people fully recover from the illness in a week.

The possible link between Zika and microcephaly is a key component of the travel warnings.

The Brazilian Ministry of Health has investigated more than 4,100 microcephaly cases in the past 13 months, according to data released Wednesday. The ministry used to see fewer than 200 cases of the condition every year, though the numbers have surged since Zika arrived in May.

The Hawaii Department of Health has said a child who was recently born with microcephaly there had been infected with Zika, and his mother likely got the virus when she lived in Brazil.

The CDC says it will also be conducting a study to examine a possible link between Zika and Guillain-Barre Syndrome, a nerve disorder than can lead to muscle weakness and paralysis. Many fully recover from the syndrome, the CDC says, though it can be fatal in rare instances.

The World Health Organization is holding an information session on Zika tomorrow.

The organization says it’s possible the virus could cause epidemics in new areas it reaches because people don’t have immunity to it. There’s also currently no vaccine to prevent it.