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FAQS about ZIKA VIRUS and INVASIVE AEDES in San Mateo County

 Zika virus has been in the news often recently, and District staff members have received many questions from the family, friends, members of the public, and the media. Here are a few of the most common questions, and their answers:

Where did Zika virus come from?

Zika virus (ZIKV) was first discovered in the 1940s in Africa, but for most of the 20th century it caused only small outbreaks in Africa and Asia. It wasn’t until 2014 that Zika virus arrived in the Americas, where it began to spread rapidly and become the epidemic we know today.

Why is Zika such a big deal?

In adults, Zika virus is usually a mild illness. The majority of people who are infected won’t even know they’re sick – although they can still pass on the virus to mosquitoes if bitten.  However, Zika virus infection during pregnancy has been linked with serious birth defects. Zika virus infection has also been linked with an autoimmune disease called Guillain-Barre Syndrome.

How is Zika virus transmitted?

Most Zika virus infections are transmitted by the bite of an infected mosquito. Only some mosquito species carry Zika virus; the primary vector in the Americas is believed to be the yellow fever mosquito, Aedes aegypti. Less commonly, Zika virus can be transmitted from mother to baby during pregnancy or birth, through unprotected sexual contact, or through blood transfusion. Zika virus is not generally transmitted from person-to-person by casual contact (e.g. shaking hands, hugging, kissing, sharing food or eating utensils).

What’s the District doing about Zika virus?

The District has been working to eradicate invasive Aedes mosquitoes in San Mateo County since the mosquito’s arrival in 2013. This program reduces not only the risk of Zika virus, but of other diseases transmitted by invasive Aedes mosquitoes, such as dengue, chikungunya, and yellow fever. The District’s aggressive eradication program includes house-to-house inspections for invasive mosquitoes, intensive mosquito population surveillance, and treatment or elimination of standing water where mosquitoes may breed. You can see the results of this program on the Aedes aegypti surveillance data page.

How worried should I be about Zika virus?

Here in San Mateo County, the risk of Zika virus infection is extremely low due to the relatively small number of human cases and the District’s successful control of invasive Aedes mosquitoes. This is not expected to change in the near future. However, travelers to areas with active transmission of Zika virus – particularly women who are or may become pregnant – should consult the latest guidance from the CDC (

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