CLIMATE CHANGE IS opening the door for disease-carrying mosquitoes to expand their range.

In worst-case scenario projections, up to a billion people could be newly exposed to mosquito-borne diseases within the next century, according to a study published Thursday in the journal PLOS Neglected Tropical Diseases.

As the world gets warmer, mosquitoes will be able to survive winters in areas they normally wouldn’t. Places such as Canada and parts of Northern Europe could start to be habitable for the insects and their diseases in the next few decades, researchers warn.

Mosquitoes are one of the deadliest animals in the world, according to the World Health Organization. Millions of people die each year from mosquito-borne diseases like malaria, yellow fever, Zika, dengue and chikungunya. In 2015, malaria alone killed more than 400,000 people.

“These diseases, which we think of as strictly tropical, have been showing up already in areas with suitable climates, such as Florida, because humans are very good at moving both bugs and their pathogens around the globe,” study author Sadie Ryan said in a statement.

Even places that only have a slight risk of becoming mosquito habitat should be concerned, researchers warn. They pointed to the Zika outbreak in Brazil in 2015 as proof that these diseases can have explosive outbreaks under the right conditions.

“Newly exposed populations tend to see erupting epidemics,” Ryan said, “and for the diseases we have seen recently, like Zika, first exposures tend to have worse outcomes, in terms of symptoms, and public health response, so we should be on the lookout for those new areas, under any future scenario.”

Researchers created models based on temperature, which is a main factor in mosquito survival and disease transmission. These models could be useful to governments and health organizations to prevent and prepare for the viruses, according to the authors.

The research examined two of the most common disease-carrying mosquitoes: Aedes aegypti and Aedes albopictus. The former would thrive under the most severe climate change predictions, but the latter would suffer in the hot conditions.

“This might sound like a good news, bad news scenario but it’s all bad news if we end up in the worst timeline for climate change,” study author Colin Carlson said in a statement. “Any scenario where a region gets too warm to transmit dengue is one where we also have different but equally severe threats in other health sectors.”