California health officials on Friday urged travelers returning home from the Olympics as well as countries where Zika is spreading to continue to wear insect repellent and to practice safe sex for several more weeks, to help prevent the spread of the virus in the Golden State.

While Zika is spread primarily through the bite of the black-and-white-striped Aedes mosquito, the virus also can be passed through sex, health officials said.

They are mostly concerned with people returning home from the Olympics in Rio de Janeiro, because Brazil has been hit hard by the virus.

“Continue using insect repellent to prevent spreading the virus to mosquitoes in your community upon your return and refrain from unprotected sex so you don’t pass the virus to your partner,” Dr. Karen Smith, director for the California Department of Public Health, said in a statement.

So far, the mosquito that carries the virus has been found in a dozen California counties, including in Los Angeles, San Bernardino and Orange counties.

There are no local transmissions reported in California, but Friday’s warning comes as Zika cases continue to climb in the state among people who have traveled abroad.

The total number of travel-associated Zika cases has jumped statewide by almost 50 percent in the last three weeks, from 114 recorded on July 29 to 170 by Friday, according to the state’s department of public health. The state began recording cases last year.

Health officials said couples should use condoms for at least eight weeks after travel. Men who have tested positive for Zika should use condoms for six months to prevent transmission to their partners, they added.

“Travelers returning from an affected region should also continue using insect repellent for three weeks to prevent the virus from spreading to mosquitoes, which might then infect others,” according to the health department.

Health officials are especially concerned about pregnant women. Two babies in California were recently born with microcephaly, a birth defect that can be caused by Zika. Both women who gave birth tested positive for the virus.

“Pregnant women and couples planning to have children need to be especially cautious because Zika can cause significant harm to a developing fetus,” Smith said. “Pregnant women who have traveled to an area with Zika should inform their doctor upon return, and couples returning from an affected area should speak with a doctor before getting pregnant.”

Two events are happening that raise concern, an increased number of Zika cases among travelers returning home to Los Angeles County and mosquito season, said Dr. Benjamin Schwartz, deputy chief of the acute communicable disease control program at the county’s public health department.

He said there could be many more cases than reported, because not everyone has symptoms.

“Something that everybody in the county should be doing is getting rid of places where mosquitoes can breed,” Schwartz said. “Dumping, draining and discarding containers where there are pools. The Aedes aegypti mosquitoes have been spreading. They are in areas in Los Angeles County seeing it for the very first time. We’re very concerned that people do their part in order to prevent transmission.”

San Diego County officials are investigating a possible local transmission of the virus. As a result, officials there plan to spray a two-block area of South Park, where they discovered the larvae from the mosquito known to carry the Zika virus, according to published reports.

Across the nation, a travel alert was issued for Florida, where there are 35 cases of local transmission of Zika. At least five 5 people contracted the virus in Miami Beach.

Like West Nile virus, Zika is primarily transmitted through the bite of an infected mosquito. Some symptoms include acute onset of fever, a flat red rash and joint pain. Although death is rare, one out of five people infected with Zika can become very ill, according to the federal Centers for Disease Control and Prevention.