Mosquitoes are among the most lethal animals in the world as a result of the dangerous viruses and organisms they carry. They cause millions of deaths every year from diseases like malaria, dengue, Zika, chikungunya and yellow fever.

Climate change is expected to make them deadlier. Researchers from four U.S. universities developed a model that projects how mosquitoes and the diseases they carry will proliferate as the planet warms. They estimate the spread of mosquitoes will cause an additional one billion infections by the end of the century. The study is published in the journal PLOS Neglected Tropical Diseases.

The researchers said warmer global temperatures could lead to new cases in regions previously not considered at risk, like the United States. The viruses can result in volatile outbreaks when the conditions are right, proliferating quickly in populations without immunity.

The study focuses on the negative effects of climate change on global health. Not only do extreme weather events threatening humans, but rising temperatures and carbon dioxide levels help mosquitoes thrive, leading to the spread of diseases.

Countries in East Africa, which already face pressure from mosquito-spread diseases, are likely to face greater pressure. These countries are especially vulnerable because they have underfunded infrastructure which cannot combat multiple health risks.