Three new cases of the Zika virus among San Francisco residents have been diagnosed in the past two weeks, the city's Department of Public Health announced Friday.
The number of San Francisco residents who have been diagnosed with the virus is now up to five. Each of the residents traveled in countries where Zika is circulating, health officials said.
The virus, which was first reported as infecting a San Francisco resident on March 3 before another case was reported April 22, is not being contracted locally, as the mosquito-borne illness is not circulating here, health officials said.
"Since it is summer travel season, we want to remind San Franciscans who are planning to travel to countries where Zika virus is circulating to protect themselves from mosquito bites," Dr. Tomas Aragon, San Francisco's Health Officer, said in a statement.
Aragon added that pregnant women in particular should avoid "unnecessary travel" to the Latin American and Caribbean countries -- and some of the Pacific Islands -- where the virus has been prevalent.
The virus causes mild symptoms of fever, joint pain, rash and red eyes. But when a mother is infected with Zika during her pregnancy, health officials said the virus is known to cause birth defects such as microcephaly, in which a child is born with an unusually small head.
Research into the exact risks posed by maternal Zika infection is ongoing, health officials said.
Recent evidence of sexual transmission of the virus from men to women and men to men is also being studied.
There is currently no vaccine available to prevent the infection, health officials said.