A rare tropical disease called Zika has made its way to the Americas recently, causing a big outbreak in Brazil and spreading to at least eight other countries.

Scientists are still learning about this disease, which has long been thought to be pretty mild. But it's getting attention because Zika has also been linked to several deaths and terrible birth defects like microcephaly, which causes babies' brains and heads to stop growing to full size.

Though it's still not known whether the virus caused any of this, the World Health Organization and its regional outfit the Pan American Health Organization (PAHO) are warning doctors and patients to be on the lookout for Zika. Here's what we know and don't know so far.

What do we know about Zika?

The Zika virus is transmitted to humans by the same type of mosquito (Aedes) that transmits the dengue and chikungunya viruses.

Zika produces very mild symptoms — rash, headaches, pain in the bones, and fever — that usually show up between three and 12 days after a mosquito bite. These symptoms usually go away within a week, and one in four people don’t even develop any symptoms after being infected with the virus.

This means people don’t usually go to the doctor for Zika, and many cases go unnoticed. There’s also no vaccine or treatment for the virus, so doctors just work on controlling and alleviating its symptoms.



Part of the reason the virus hasn't gotten a lot of attention is that the mortality rate for Zika is also extremely low, and researchers don't think of it as a very deadly virus. Though Zika was first discovered in 1947 (in the Zika forest in Uganda), it hasn't bothered humans much in all these years. "There have only been about 14 or 15 cases documented until 2007," said Dr. Marcos Espinal, the director of communicable diseases at PAHO. That's when there was the first big Zika outbreak, reported in the Yap Island in Micronesia. Other Pacific Islands — Fiji, Vanuatu — and Australia have had periodic outbreaks since.

In 2014, the virus made an appearance in the Americas, arriving in the Easter Islands, and since then it’s shown up in nine countries, including Mexico and Brazil.

On December 1, PAHO issued an alert about the threat to the Americas, asking public health officials and doctors to be on the lookout for cases.

There's a risk that people or mosquitoes carrying the disease will come into the US, so you might be hearing more about Zika in the news very soon.

What don’t we know?

Because it's so rare and hard to detect, there's still a lot to learn about Zika.

The most pressing question about the virus is whether it causes serious developmental disorders like microcephaly, the congenital condition that's associated with a small head and incomplete brain development.

There's a bit of panic about this link in Brazil right now because some of the regions with the most Zika cases have also seen an uptick in microcephaly births.

Espinal said that researchers from the CDC and the Brazilian health authorities are investigating the link, but right now it's unclear whether the two are related. He explained that microcephaly can be caused by many different factors — including other viruses or Down syndrome — and researchers still don't know whether Zika can even cross the placental barrier in mothers.

"There’s an ecological link there, but causality is very difficult to determine and we need more research," he added.

There’s a chance that Zika causes Guillain-Barré syndrome, a very rare neurological syndrome that's been showing up in some Zika patients. But again, all researchers have now is a potential link — and more research needs to be done.

Finally, there may be more to learn about how deadly Zika can be. Researchers have long believed Zika is a mild virus. (As one microbiologist put it, it's "a lamb, not a lion.") But the virus has been linked to at least seven deaths in Brazil as of November 30. Again, correlation is not causation. As Espinal said, "Zika has a very, very low mortality rate." But it's possible this virus is deadlier than scientists previously thought.

Why are people talking about Zika now?

Because of the current outbreak in Latin America. In February 2014, Zika was detected in Chile on Easter Island. In May 2015, Brazil confirmed its own cases. As of this October, there have been more than 1,200 cases in 14 states in Brazil. Colombia, El Salvador, Guatemala, Mexico, Paraguay, Panama, Suriname, and Venezuela have recently reported cases, too.

So that’s why the PAHO just issued an alert about the threat, telling countries to be on the watch for the disease and any unusual birth patterns with babies.

Only about 1,000 of the 60,000 cases so far have been confirmed, said Espinal, "so it's important to strengthen surveillance for Zika — to do surveillance and look for congenital abnormalities." Countries are also being advised to take measures to destroy mosquito breeding sites and raise public awareness about the virus.

Is it true moms in Brazil should avoid having babies?

There's a lot of panic in the news right now, especially in Brazil. Some local health officials have reportedly suggested that women there avoid getting pregnant until more is known about Zika.

Espinal said there's just not enough evidence right now to justify avoiding pregnancy. PAHO is recommending pregnant women follow up with their doctors but nothing more drastic. "There’s no need to panic. We need to evaluate the data because we don't know the answers to many questions about Zika. We don't even know if the virus crosses the placental barrier."

How can you protect yourself?

There are no travel restrictions for countries experiencing Zika outbreaks, but public health officials are suggesting the following for those in affected regions: