Zika virus is transmitted by Aedes aegypti mosquitoes (also known as yellow fever mosquitoes) and Aedes albopictus mosquitoes (also known as Asian tiger mosquitoes). These mosquitoes are not native to California. However, since 2011 they have been detected in several California counties. An Aedes mosquito can only transmit Zika virus after it bites a person who has this virus in their blood. Thus far in California, Zika virus infections have been documented only in people who were infected while traveling outside the United States or through sexual contact with an infected traveler. To date there has been no local mosquito-borne transmission of Zika virus in California.
Zika virus is not spread through casual contact, but can be spread by infected persons to their sexual partners. Zika virus infection in pregnant women can cause fetal microcephaly (abnormally small head and brain) and other poor pregnancy outcomes. Additionally, there is an association between Zika and Guillain-Barré Syndrome (GBS), a disease affecting the nervous system.