Zika, which the World Health Organization says is "spreading explosively" across the Americas, is transmitted by the same mosquitoes that also carry other tropical viruses such as dengue and yellow fever. Global health officials are alarmed because of its potential link to brain defects in infants as well as a rare syndrome that can lead to paralysis.

Here's what you need to know about Zika:

- The WHO will convene a special meeting on Monday to determine whether to declare a global public health emergency like Ebola. What is this and what does this trigger?

READ: Zika 'spreading explosively', emergency meeting Feb 1: WHO chief

Technically known as a Public Health Emergency of International Concern (PHEIC) declaration, it is defined as "an extraordinary event which is determined to constitute a public health risk to other States through the international spread of disease and to potentially require a coordinated international response." In the past it has been used only in the most dire of circumstances.

The last time the WHO took this action was in August 2014 with Ebola and it is typically done when there is a great need for a coordinated international response deemed essential to stopping the spread of a disease. The WHO has done this only twice before - during the 2009 during the H1N1 influenza epidemic and in May 2014 regarding the re-emergence of polio.

The declaration typically comes with a list of global recommendations to nations regarding practical measures such as international travel, trade and border crossings, as well as areas of scientific inquiry to target. The formal declaration was essential to mobilizing resources for previous crises by persuading wealthier countries to send more health workers and supplies to the hardest-hit regions.

What is Zika?

A summary of the history of the virus from the Post's Health section:

The virus was discovered in 1947 in a feverish rhesus monkey living in the Zika Forest of Uganda, but until 2007 scientists knew of only 14 human cases of the disease. That year it arrived on the travel-brochure-perfect Yap Island in the southwestern Pacific Ocean. Within a few months, nearly three-quarters of the island's 11,000 or so residents older than 3 had been infected.

At first, those sick with Zika developed fever, joint pains and eye inflammation; then a red, bumplike rash erupted, sometimes followed by painful swelling of hands and feet. Some people vomited. Others were sensitive to light. But the symptoms usually resolved a couple of days later, and no one died.

In 2013, Zika popped up again, this time in Tahiti and other parts of French Polynesia. An estimated 28,000 people (about 11 percent of the population of those islands) felt sick enough with the virus to seek medical care. By 2014 it was showing up in several other South Pacific spots: New Caledonia, east of Australia; the Cook Islands; and, early this year, Easter Island, which marked the official arrival of the disease in the Americas, since that remote island is part of Chile.

Zika showed up in Brazil in May.


What's going on in Brazil?

The country is facing an unprecedented number of Zika virus cases - more than 1 million - as well as an unprecedented number of microcephaly cases that health officials believe are linked. In recent months, Brazil has spent more than $300 million to battle the mosquito, mobilizing hundreds of soldiers in the effort and going door-to-door to try to wipe out places where mosquitoes breed.

According to correspondent Dom Phillips, the Brazilian Health Ministry says 80 percent of those who catch Zika show no symptoms. The rest may suffer fever, muscle pain and rashes for a few days. Most people who come down with it recover quickly.


"We never paid too much attention to this virus," said Paulo Zanotto, a microbiology professor at the University of Sao Paulo who is coordinating a network of 42 laboratories studying Zika. "I'm really worried because we have no idea of the amount of spread."

The government estimates that there are between 400,000 and 1.4 million Zika cases in the country but there has been some confusion about the number of microcephaly cases. At one point Brazil said there were 4,180 suspected cases. However, on Wednesday, the country said it had looked in to 700 of those and that 270 had been confirmed and 462 had been ruled out.

READ: Brazil declares emergency after 2,400 babies are born with brain damage

In 2014, Brazil reported only 150 cases, which is a very small number for its population.


What's the scientific basis for the suspected link between Zika and microcephaly?

The World Health Organization and the Centers for Disease Control and Prevention, which is assisting Brazilian authorities in their investigation, have yet to definitively establish a connection between Zika and microcephaly. But the CDC has confirmed the presence of Zika in the bodies of two newborns with microcephaly who died and in the placentas of two women who miscarried children with microcephaly.

On Thursday, Brazil's health minister, Claudio Maierovitch, said the country is investigating 12 confirmed deaths of babies born with microcephaly for potential linkage with Zika virus infection. The country has more than 4,000 suspected cases of microcephaly. He said pregnant women who tested positive for Zika virus have had a rash and fever during the first and second trimesters.


What other complications related to Zika should I be worried about?

The CDC is working with Brazilian health officials to investigate a possible link to a growing number of cases of Guillain-Barré syndrome, a rare disease that occurs when your own immune system damages nerve cells. Most people recover fully from it, but others experience long-term nerve damage or paralysis. In rare cases, people have died.

In Brazil, health officials have identified hundreds of cases of Guillain-Barré, a significant increase from last year.


What do we know about the cases in the United States?

Health officials said Zika virus may be the reason for a baby born in Hawaii with microcephaly - the first such case in the country. The mother is believed to have contracted the disease while living in Brazil last spring.

Florida, Illinois, New Jersey, Texas, Arkansas and other several other states have confirmed that residents who recently traveled to countries where Zika virus has been found in mosquitoes have tested positive for the virus. Another 20 have been confirmed to have the virus in Puerto Rico and the U.S. Virgin Islands, bringing the total number in the United States to 51.


How is the virus transmitted?

When a mosquito bites an infected person and then bites someone else. It is cannot be transmitted from person to person.

Haiti said on Jan. 15 that it had confirmed its first cases of Zika virus in five people in the area of Port-au-Prince, the country's capital. There were cases last year in Panama and Puerto Rico.

PHOTO ALBUM: Brazil: Zika birth defects

Colombian officials said Wednesday that 13,500 people are infected already and that the number could grow rapidly.

"We expect an expansion similar to what we had with the chikungunya virus last year, to finish with between 600,000 to 700,000 cases," health minister Alejandro Gaviria told journalists, according to Reuters.


How vulnerable is the United States?

Some infectious disease experts believe it's only a matter of time before mosquitoes with the virus make their way to the United States. They believe Zika is likely to follow a path similar to dengue fever in the United States, with outbreaks beginning in Puerto Rico and Florida and spreading across the Gulf states.

The nation's Gulf Coast presents an ideal setting for the virus because of its warm climate and the multiple species of mosquitoes there that can transmit the disease - including A. aegypti, which carries Zika. The region also has another risk factor: poverty. In poorer parts of the South, many residents still live without window screens or air conditioning and in proximity to stagnant water sources that are ideal mosquito breeding grounds.

Yet much of the entire country could be vulnerable. Scientists believe the virus could spread northward due to warming weather.


What can I do to reduce my risk of becoming infected or sick?

The CDC has warned pregnant Americans to try to avoid traveling to certain countries. Its initial list contained 14 countries, but the CDC on Friday added eight more - in South America, the Caribbean and Polynesia - as places where the reach of the virus is growing.

According to the CDC guidance:

"Zika virus can be spread from a pregnant woman to her unborn baby. There have been reports of a serious birth defect of the brain called microcephaly and other poor pregnancy outcomes in babies of mothers who were infected with Zika virus while pregnant. Knowledge of the link between Zika and these outcomes is evolving, but until more is known, CDC recommends special precautions for the following groups:

"Women who are pregnant (in any trimester): Consider postponing travel to any area where Zika virus transmission is ongoing. If you must travel to one of these areas, talk to your doctor first and strictly follow steps to prevent mosquito bites during your trip.

"Women who are trying to become pregnant: Before you travel, talk to your doctor about your plans to become pregnant and the risk of Zika virus infection. Strictly follow steps to prevent mosquito bites during your trip. Specific areas where Zika virus transmission is ongoing are often difficult to determine and are likely to change over time."

As of Thursday, here are the countries and territories impacted, according to the Pan American Health Organization:

Brazil, Colombia, El Salvador, French Guiana, Guatemala, Guyana, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Saint Martin, Suriname and Venezuela, as well as Puerto Rico.

On Friday, the CDC added eight more countries to the list:

Barbados, Bolivia, Ecuador, Guadeloupe, Saint Martin, Guyana, Cape Verde, and Samoa.

And this week, the agency also added the U.S Virgin Islands and Dominican Republic to the list.

If you're traveling to one of these countries, here are some tips from the CDC for travelers to prevent getting bitten by mosquitoes:

- Wear long-sleeved shirts and long pants.

- Use insect repellent approved by the Environmental Protection Agency as directed.

- Use products with a higher percentage of the following ingredients: DEET (products include Off!, Cutter, Sawyer and Ultrathon), Picaridin, also known as KBR 3023, Bayrepel, and icaridin (products include Cutter Advanced and Skin So Soft Bug Guard Plus, as well as Autan outside of the United States), Oil of lemon eucalyptus (OLE) or PMD (products include Repel), or IR3535 (products include Skin So Soft Bug Guard Plus Expedition and SkinSmart).