Everything you ever wanted to know about the Zika virus and its spread across North and South America. (Daron Taylor,Claritza Jimenez/The Washington Post)

The World Health Organization has designated the Zika virus a public health emergency of international concern, an action it has taken only three times before and which paves the way for the mobilization of more funding and manpower to fight the mosquito-born pathogen spreading "explosively" through the Americas.

On Monday, the Centers for Disease Control and Prevention added four more countries to the list of those pregnant women should try to avoid. They are American Samoa, Costa Rica, Curacao  and Nicaragua. The full list can be found here.

The virus is transmitted by the same mosquitoes that 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:

What is a global public health emergency? The WHO convened a special meeting on Monday to determine whether to declare a global public health emergency like the one for Ebola. What is this and what does this trigger?

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 had done this only twice previously — during the 2009 during the H1N1 influenza epidemic and in May 2014 regarding the reemergence 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.

I just head about the patient in Dallas who may have contracted Zika through a sexual partner rather than a mosquito bite. How worried should I be about this?

Dallas officials said on Feb. 2 that a person who had not traveled outside the United States appears to have been infected by a sexual partner. The U.S. Centers for Disease Control and Prevention confirmed the person has contracted the virus, but has not explicitly said the transmission was through sex. However, the CDC updated its advisory for pregnant women to reflect this concern. The CDC says that "until more is known," pregnant women should "avoid exposure to semen from someone who has been exposed to Zika virus."

In Britain, health officials said that if a female is at risk of getting pregnant or already pregnant her partner should use a condom if he has been traveling under the following conditions: "for 28 days after his return from an active Zika transmission area if he had no symptoms of unexplained fever and rash" and "for 6 months following recovery if a clinical illness compatible with Zika virus infection or laboratory confirmed Zika virus infection was reported."

What about blood transfusions? 

This is a very real concern. Experts say it is theoretically possible to get Zika through blood transfusions from an infected donor, given that they have seen transmission of other related mosquito-borne viruses through the blood. On Feb. 3, health officials in one city in Brazil reported that they believe two people may have contracted the virus from transfusions, but this has not been confirmed by national authorities, the World Health Organization or other agencies.

Meanwhile, health officials in a growing number of countries are taking precautions to prevent contamination of the blood supply by urging potential donors to delay giving blood for about a month if they have been traveling in Zika-affected regions.

(This video was updated on Feb. 2, 2016.) Authorities have confirmed more than 30 cases of Zika virus in the United States. Here's what you need to know. (Gillian Brockell/The Washington Post)

What is Zika?

A summary of the history of the virus from our 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 Washington Post correspondent Dom Phillips, the Brazilian Health Ministry has said 80 percent of those who catch Zika have shown no symptoms. The rest may suffer fever, muscle pain and rashes for a few days, and most people who come down with it recovery 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. But on Wednesday, the country said that it had looked into 700 of those and that 270 had been confirmed and 462 had been ruled out.

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, Claudio Maierovitch, director of the department of surveillance of communicable diseases at Brazil’s Health Ministry, 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 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. The Associated Press reported that the state's epidemiologist, Sarah Park, said the mother no longer had the virus when she arrived in Hawaii and neither did the boy, so there is no chance a mosquito could have bitten them and spread it.

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. As of Feb. 4, a total of 51 cases have been reported in 14 states and the District of Columbia. Another 21 have been confirmed to have the virus in Puerto Rico (20) and the U.S. Virgin Islands (1). That brings the total number of cases in the United States to 71. More cases are likely to be reported as residents travel to Zika-affected regions.

How is the virus transmitted?

The primary way this virus is spread happens when a mosquito bites an infected person and then bites someone else. It cannot be transmitted from casual contact person-to-person. Before the report of the Dallas patient who contracted Zika through a sexual partner, there was only one other reported case of sexual transmission.

On Feb. 3, local health authorities in Brazil said two people may have contracted Zika through blood transfusions, the first such cases reported.

The map above shows Zika spreading across the globe. How close is it to us?

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.

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 counties 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).

Brady Dennis contributed to this report.

This post has been updated.

Read more:

WHO calls emergency meeting, says Zika virus ‘spreading explosively' across Americas

Why the United States is so vulnerable to the alarming spread of Zika virus

For more health news, you can sign up for our weekly newsletter here.