What you need to know about Zika

Concerns are growing over the mosquito-borne illness known as Zika virus, which has been spreading through Central and South America and is believed to be linked to a surge in serious birth defects in Brazil and more recently paralysis from Guillain-Barré syndrome.

The U.S. Centers for Disease Control and Prevention has issued a travel advisory urging pregnant women to avoid travel to more than two dozen countries and territories, mostly in Latin America and the Caribbean, where Zika virus is present. Because of the possible link to birth defects, pregnant women who must travel to affected areas should talk to their doctor or other health care provider first and strictly follow steps to avoid mosquito bites during the trip, the CDC said.

The virus reached Mexico in November and Puerto Rico in December, and the CDC has confirmed more than 50 cases of Zika in the U.S., all but one in travelers who recently returned from trips to Latin America. In one case, a person who caught it abroad transmitted it to their partner through sex.

Here's a primer on what you should know about the disease.

What is Zika virus?

Zika virus is an illness transmitted to people through bites from mosquitoes of the Aedes species -- the same mosquitoes that spread dengue and chikungunya viruses. It not communicable from person to person but can be transmitted when a mosquito bites someone who's infected and then bites someone else. The virus was first discovered in Uganda in 1947 and named after the forest in which it was found.

Officials say the current Zika outbreak in Brazil began last May. Authorities there estimate that since then, between 440,000 and 1.3 million people have caught it. Zika has spread to other countries in Latin America and the Caribbean, including Colombia, Venezuela, Honduras and Mexico. Puerto Rico reported its first case of locally transmitted Zika virus in December.

What are the symptoms?

According to the CDC, the most common symptoms of Zika virus are fever, rash, joint pain, and conjunctivitis. Other symptoms can include muscle pain, headache, pain behind the eyes, and vomiting. Symptoms are usually mild, lasting from a few days to a week. Many people infected with the virus experience no symptoms at all. In rare cases, symptoms can become severe and require hospitalization.

A number of Zika patients in Brazil have also gone on to develop a rare autoimmune condition called Guillain-Barré syndrome, which can cause at least temporary paralysis. Health officials are investigating the connection.

Is there a vaccine or cure?

There is no vaccine to prevent Zika virus. The U.S. National Institutes of Health is ramping up efforts to develop one, but the process will take time.

"It is important to understand we will not have a widely available safe and effective Zika vaccine this year and probably not in the next few years," said Dr. Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases. There is no specific treatment for Zika except to try to ease the symptoms.

What do we know about its possible link to birth defects?

Health officials in Brazil say they've found strong evidence that Zika has been linked to a sudden rise in the number of babies being born with abnormally small heads, a condition called microcephaly, which usually results in mental retardation. Brazil's government reports more than 4,000 babies have been born with microcephaly since the Zika outbreak began there, up from fewer than 150 in 2014.

Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, explained how the connection was found.

"First we saw a dramatic rise in microcephaly in Brazil coinciding with when Zika was introduced there. This prompted an active search to see if there was a virus behind it and Zika was one of the suspect viruses, even though it was not shown previously to cause congenital birth defects," he told CBS News. "The researchers there took blood samples and other tissue samples from these babies with microcephaly and found evidence of the virus in the samples. They also sampled the amniotic fluid of mothers who had babies with microcephaly and that really helped confirm the connection."

While more research is needed to confirm and to find treatment, researchers say the evidence to support the link is very strong. In response, authorities in Brazil, El Salvador and some other affected areas have told women to put off pregnancy if they can.