Federal health officials today said they believe "thousands" of people may have contracted the Zika virus before returning to the U.S. as they remain concerned that the virus might start to have ongoing transmission in the U.S.

Speaking at a panel at the Woodrow Wilson International Center for Scholars, Dr. Anne Schuchat, principal deputy director at the Centers for Disease Control and Prevention, said the Zika virus remains "pretty concerning" for experts as they learn how it affects pregnant women.

"The reality is one bite, and if you’re pregnant, your baby might be harmed," Schuchat said at the panel today. "That’s a phenomenal problem."

Common symptoms of the Zika virus include fever, rash, joint pain and conjunctivitis, according to the CDC. Approximately one in five people infected with the virus show symptoms. Severe complications from the virus that require hospitalization are rare, according to the CDC.

The virus has also been linked to the serious birth defect microcephaly. The birth defect is characterized by a malformed or smaller head and brain, and can result in serious developmental delays.

At today's panel government, health officials said they are concerned about local transmission of the virus if travelers spread the virus to mosquitoes in the U.S., which can then infect other people who have not traveled abroad where Zika transmission is ongoing.

Schuchat said approximately 500 people in the U.S. were found to have likely been infected with Zika. However, since 80 percent of people with a Zika infection do not show symptoms, she estimates that thousands may have arrived in the U.S. unaware they were infected with the Zika virus and potentially able to start an outbreak through the mosquito population.

She explained this number is especially concerning since local mosquito control has diminished in recent years.

"We’re not starting in a good place. We used to have a lot stronger mosquito control and mosquito surveillance," said Schuchat. "We really have a patchwork nation around mosquito capacity. The local governments are really concerned."

 

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Dr. Anthony Fauci, director of the National Institute of Allergy andInfectious Diseases at the National Institutes of Health, said at the panel that the Aedes aegypti mosquito, the main type of mosquito that spreads Zika and other diseases, is notoriously difficult to kill. He added that he expects to see some local transmission of the virus in the same way there were limited outbreaks of the Dengue virus and Chickungunya virus.

"History has told us this is a really difficult mosquito to deal with," said Fauci, adding that a mosquito-based outbreak is far different from an outbreak that spreads from person to person. "It’s a whole new venue of transmission."

http://abcnews.go.com/Health/cdc-official-thousands-arrived-us-zika/story?id=39332409