How far could Zika spread?

Before the end of last year, the Zika virus was virtually unknown.

But when nearly 3,000 babies in Brazil were born with undersized heads last year, the Brazilian government connected the births to a surge of cases of Zika that occurred earlier that year. This condition, called microencephaly, can kill newborns or leave them mentally and physically disabled for life, when expectant mothers contract the disease during pregnancy. (Zika produces relatively mild symptoms in adults.)

Though the original outbreak began in Brazil, 22 other countries in Central and South America, including El Salvador and Mexico, have reported active transmission of the virus. Cases have also been reported in the US and the UK, in travelers who visited areas with Zika present.

Today (Jan. 25), the World Health Organization’s Regional Office for the Americas released a statement saying “that Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found.”

There are two types of Aedes mosquito capable of transmitting the virus. In most cases, Zika spreads through the Aedes aegypti mosquito—the same mosquito that spreads Dengue fever, the Chikungunya virus, and yellow fever. However, a cousin of the aegypti mosquito, called Aedes albopictus, can also pass along the disease.

If albopictus mosquitoes carry more Zika more efficiently than aegypti, the virus could reach a much broader range than their current tropical region, says Peter Armbruster, a biologist at Georgetown University studying the species of mosquito. At present albopictus isn’t the primary disease vector for Zika, but it could become one if the virus mutates like other viruses have done in the past.

Armbruster explained that unlike the aeypti mosquito, albopictus is an invasive species that can make its home in almost any place where there’s a standing body of water. Since they first were discovered in a port in Houston, Texas in 1985, albopictus mosquitoes have spread to almost 1,000 miles north in the United States. They’ve made it to “every continent except Antarctica,” Armbruster told Quartz. “This kind of invasion and range expansion is somewhat a natural experiment in evolution.”

International travelers could carry Zika to regions where there are albopictus, leading to outbreaks in atypical areas, which is what happened with Chikungunya. In 2007, travelers brought back a strain of the virus to northern Italy. The albopictus mosquito, which also lives in that region, spread the virus to create a minor outbreak.

“We typically think of Chikungunya as a tropical virus, and this was the first time there had been an outbreak of this virus in a temperate region,” says Armbruster. “The question becomes, ‘Is there any reason that something like that couldn’t happen in D.C. or Baltimore?’ I can’t think of a really compelling reason for why not.”

So far, it’s hard to determine whether or not all the conditions for an outbreak will be right for albopictus to carry Zika the way aegypti does. “It’s hard to say [that the conditions are] totally implausible, but on the other hand they might be rare, and it’s just really hard to happen,” says Armbruster. “It’s just really hard to predict.”