CDC Official Says Thousands May Have Arrived in US With Zika Federal officials said they're concerned about local transmission of the virus.

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

Speaking at a panel at the Woodrow Wilson International Center for Scholars, Dr. Anne Schuchat, the 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 Zika infection include fever, rash, joint pain and conjunctivitis, according to the CDC. Approximately 1 in 5 people infected with the virus shows symptoms. Severe complications from Zika infection that require hospitalization are rare, according to the CDC.

The virus has been linked to the serious birth defect microcephaly, which 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 Zika if travelers spread the virus to mosquitoes in the U.S., which can then infect other people who have not traveled to countries with Zika epidemics.

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 estimated that thousands may have arrived in the U.S. unaware they were infected with the virus and were potentially able to start a local outbreak through the mosquito population.

She explained this number is especially concerning because 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."

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, said at the panel that the Aedes aegypti mosquito, the main mosquito species 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 have been limited outbreaks of dengue fever and chikungunya infection.

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