As the World Health Organization decides whether to declare the Zika virus outbreak a global public health emergency, here's a look at other pandemics that made it to our shores. (Gillian Brockell/The Washington Post)

With the Zika virus now circulating in two dozen countries and territories across the Americas, the mosquito-borne pathogen seems destined to reach the United States, and probably sooner rather than later.

What is far less certain, say public health and infectious disease experts, is Zika’s potential reach and impact here. The South is seen as especially vulnerable because of its warm, humid climate and pockets of poverty where more people live without air conditioning or proper window screens. Plus, the region is already home to mosquitoes that can transmit the virus.

Some models estimate as many as 200 million people live in areas that might be conducive to the spread of Zika during summer months — including the East and West coasts and much of the Midwest. That makes for a huge target as researchers scramble to determine how the virus manifests itself in the human body, who is particularly at risk, and why.

Their urgency comes amid reports from Brazil, the epicenter of Zika, of thousands of newborns with microcephaly. The rare condition involves an abnormally small head and brain. Brazilian doctors have also seen a surge in another rare syndrome, Guillain-Barré, which can lead to paralysis. With both issues, a link to Zika is suspected.

On Thursday, the World Health Organiation announced it will convene an emergency meeting on how to stop transmission of the virus, which officials said is “spreading explosively” in the Americas.

“The level of alarm is extremely high, as is the level of uncertainty,” said Margaret Chan, WHO’s director-general. Chan has called the circumstantial evidence connecting Zika to microcephaly “extremely worrisome.”

Already, Brazil, Colombia, El Salvador and Jamaica have urged women to postpone pregnancy. The Centers for Disease Control and Prevention has an ever-expanding advisory urging U.S. women who are pregnant to avoid traveling to those 24 countries and territories in the Caribbean and Central and South America that have local transmission.

President Obama received a briefing Tuesday about the situation from his health and national security teams, including Health and Human Services Secretary Sylvia Mathews Burwell, CDC Director Thomas Frieden and Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

So far, experts are divided about what lies ahead for the United States. Nearly three dozen cases have been confirmed to date in 11 states and the District of Columbia, but CDC acknowledges that the number is growing rapidly. In each, the person was believed to have been infected while out of the country.

Fauci is playing down the potential for a significant eruption of Zika here. He notes that dengue and chikungunya, diseases transmitted by the same kinds of mosquitoes, are widespread in Latin America. But their foothold in the United States has been controlled, with only small clusters of cases.

“It is unlikely that we will have a major outbreak of Zika in this country,” Fauci said.

Yet, Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, has fears. He recalls spending much of 2014 insisting that the Ebola outbreak would not become a significant problem in the United States. But he gives no such assurance now.

“I think we’re in for real trouble in the United States,” he said, considering how swiftly Zika can spread. He focuses on conditions throughout the Gulf Coast, where stagnant water sources — in uncollected garbage, discarded tires, untended bird baths — can be ideal breeding grounds for mosquitoes much of the year.

“You’ve got to assume the worst-case scenario,” Hotez said.

Aedes albopictus, the Asian tiger mosquito, which can transmit the virus, is present from Florida to Connecticut and as far west as Illinois, said Laura Harrington, chair of Cornell University’s entomology department. It bites people as well as animals. A second species, Aedes aegypti, the yellow-fever mosquito, has a more limited geographic footprint and bites only people. But it is the primary “vector” for Zika’s transmission.

Taken together, “there are many parts of the United States that are vulnerable because of where the mosquito populations are,” Harrington said.

The experts acknowledge that projections are hampered by a limited knowledge about the disease. It takes 10 to 11 days for a mosquito to become fully infected and able to pass on the virus, according to Nikos Vasilakis, an arbovirologist at University of Texas Medical Branch in Galveston, where researchers are working in collaboration with scientists at Yale’s School of Public Health in Brazil. They don’t yet know what the risk is for pregnant women who are bitten by an infected mosquito and contract the virus, much less what the risk is for their fetuses.

The CDC has issued guidelines for pregnant women and newborns. For example, it recommends testing for the virus when babies have microcephaly and their mothers traveled to or lived in Zika-virus regions while pregnant. On Tuesday, agency officials held a televised briefing with clinicians nationwide to go over guidelines as part of increased vigilance and public education.

In the Brazilian cases, most of the exposure to Zika appears to have been during the first trimester of pregnancy. But CDC officials have said there is also evidence that a fetus can be in danger into the second trimester.

There is no drug to treat Zika or a vaccine to prevent it, although Fauci said researchers are working on the latter. Common symptoms of infection are fever, rash, joint pain and red, itchy eyes. Symptoms are usually mild and last several days to a week.

One of the biggest problems in researching the possible link between Zika and cases of birth defects has to do with the widespread presence of dengue disease in the countries with high Zika infection, said Lyle Petersen, director of CDC’s vector-borne disease division. Humans produce antibodies in response to both viruses, but current tests cannot differentiate between them.

“In people with previous dengue exposure, there’s no test to be able to sort that out,” Petersen explained.

Past outbreaks of Zika occurred among small populations in the Pacific islands, and uncommon kinds of birth defects were not noticed right away. After Brazil began reporting increased prevalence of microcephaly last October, authorities in French Polynesia went back and analyzed their 2013-2014 Zika outbreak involving more than 30,000 people. They found an increase in microcephaly cases and of Guillain-Barré syndrome.

Brazil has said it wants to expand an experiment using genetically modified mosquitoes produced by a British company called Oxitec. The company says they can be effective at controlling pest populations: Altered male mosquitoes are released into the wild to mate with females. The offspring never reach adulthood, blocking the next generation from carrying diseases such as dengue, chikungunya and Zika virus.

Oxitec says that trials involving its mosquitoes in Brazil, Panama and the Cayman Islands “resulted in a greater than 90 percent suppression” of the Aedes aegypti population. This month, the company announced that the city of Piracicaba, Brazil, would expand its effort to eradicate disease-­carrying mosquitoes. But it’s unlikely that genetically engineered mosquitoes will be buzzing around the United States anytime soon.

Beth Ranson, a spokeswoman for the Florida Keys Mosquito Control District, which has partnered with Oxitec for a proposed trial, said that before any altered bugs could be released there, the Food and Drug Administration must complete an environment assessment. Its findings must then be opened to public comment. Once finalized, a local board would need to approve the trial — over the expected opposition of some residents.

In Harris County, Tex., which includes Houston, mosquito control director Mustapha Debboun said that if local transmission of Zika occurred, workers with handheld equipment would probably begin “target spraying” small ­areas where mosquitoes breed.

“We all have to do something. This darn virus is spreading fast,” he said. “We don’t want it to get established in the United States.”