The Zika virus is a mosquito-transmitted infection related to dengue, yellow fever and West Nile virus. It did not begin spreading widely in the Western Hemisphere until last May, when an outbreak occurred in Brazil.

Experts believe that the vast majority of all Zika infections are transmitted by mosquitoes, not sex. As of Feb. 2, there have only been three reports suggesting sexual transmission.

The infection caused no symptoms and leads to no lasting harm. Scientific concern is focused on women who become infected while pregnant after exposure to the Zika virus. 

The CDC has issued tentative new guidelines suggesting that pregnant women avoid contact with men who have recently returned from areas with Zika transmission. Health authorities have suggested using condoms for at least 28 days. 

The possibility that the Zika virus causes microcephaly -unusually small heads and often damaged brains – emerged only in October, when doctors in northern Brazil noticed a surge in babies with the condition. Circumstantial evidence suggest that the Zika virus is the main cause. 

It is not known how common microcephaly has become in Brazil’s outbreak. About 150 cases of microcephaly are reported.

Zika virus is often a silent infection, and is hard to diagnose. Until recently, Zika was not considered a major threat because its symptoms are relatively mild. Only one of five people infected with the virus develop symptoms, which can include fever, rash, joint pain and red eyes and do not have to be hospitalized. 

To detect Zika, a blood or tissue sample from the first week in the infection must be sent to an advanced laboratory so the virus can be detected through sophisticated molecular testing. 

Health agencies hold to a variety of estimates for the Zika virus’ incubation period, but all are between two and 12 days after the mosquito bite. Most people start feeling sick between the third and seventh day. People who get sick usually recover within seven days. 

Federal health officials say that newborns should be tested if their mothers have visited or lived in any country experiencing an outbreak and if the mothers’ own tests are positive or inconclusive. The reason, officials said, is that infection with the virus could be linked to defects in vision and hearing, among other abnormalities, even if the child does not suffer microcephaly. The other defects may require further assessments and testing. The CDC does not recommend medication for people infected with the Zika virus. The symptoms are mild – when they appear at all – and usually require only rest, nourishment and other supportive care. 

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