The World Health Organization is asking governments for another $4 million US to fight the Zika virus.

The United Nations health agency said Tuesday that when Zika infection happens, on average there's a two or three week period before neurological complications such as Guillain-Barre Syndrome may occur that lead to muscle weakness. In severe cases, patients could experience trouble breathing and need intensive care.

Zika and other viruses are associated with Guillain-Barre Syndrome. It has not been proven as a cause. Health officials advise everyone in affected areas to take precautions against mosquito bites to prevent infection with Zika, dengue and other mosquito-borne viruses.

When pregnant women are infected with Zika virus, babies may be born with microcephaly or small heads and other brain abnormalities.

In Brazil, clusters of babies were born with microcephaly after their mothers were infected with Zika virus in the first trimester. Doctors and scientists are working to estimate the proportion of Brazilian babies suspected of having microcephaly based on head circumference at birth and those who have gone on to be confirmed cases based on clinical exams.

The associations between Zika virus, microcephaly and neurological complications as well as reports of cases of sexual transmission of the virus from travellers expands the risk groups beyond women of childbearing age, WHO Director General Margaret Chan told reporters.

"The more we know, the worse things look," Chan said at a news conference in Geneva.

The WHO and its Pan American Health Organization asked for $25 million US to fight Zika. Chan said $3 million US has been received and it is in "active discussion" for another $4 million US.

Chan said she's borrowing from the flexible 20 per cent portion of WHO's budget so far because the public health implications of Zika, such as caring and following affected babies, are so huge. But she's concerned about how sustainable that is.

The funds are being used to send experts to Cape Verde off the coast of West Africa, which does not have diagnostics to test for Zika infection, Chan said, as well as to organize scientific meetings on the virus and to build the knowledge base about it.

Elsewhere on Tuesday, the U.S. Centers for Disease Control and Prevention reminded healthcare workers to use standard procedures such as frequent handwashing and wearing masks when delivering babies to prevent possible infection with Zika or transmission of the virus to newborns.

Since most people infected with Zika do not have symptoms, the precautions should be standard and are already used to prevent transmission of HIV and hepatitis C.