Increased threats from infectious diseases stem directly from climate change, says Dr. Aaron Bernstein, interim director of Harvard University's Center for Climate, Health, and the Global Environment.

"Climate change has already made conditions more favorable to the spread of some infectious diseases," he said on the center's website, "including Lyme disease, waterborne diseases such as Vibrio parahaemolyticus, which causes vomiting and diarrhea, and mosquito-borne diseases such as malaria and dengue fever."

Bernstein, a pediatrician at Boston Children's Hospital, made the comment as part of a series of answers he posted to questions the center has received regarding the environment and COVID-19.

Temperature changes allow pathogens to move into new areas. Last July, for example, we reported on a medical journal warning that flesh-eating ocean bacteria appeared to be increasing their range northward up the Atlantic Coast as climate change warmed the water.

"Climate change definitely has an impact on infectious diseases," Elke Hertig, who studies climate and health at the University of Augsburg in Germany, told New Scientist. In her research, she has found that warmer temperatures will help malaria-carrying mosquitoes spread northward across Europe but also will make some Mediterranean areas too hot and dry for them.

"As the planet heats up, animals big and small, on land and in the sea, are headed to the poles to get out of the heat," Bernstein said. "That means animals are coming into contact with other animals they normally wouldn't, and that creates an opportunity for pathogens to get into new hosts."

Deforestation also forces animals into new habitats by destroying their homes. "The recent Ebola epidemic in West Africa probably occurred in part because bats, which carried the disease, had been forced to move into new habitats because the forests they used to live in had been cut down to grow palm oil trees," Bernstein said.

"Historically, we have grown as a species in partnership with the plants and animals we live with. So when we change the rules of the game by drastically changing the climate and life on earth, we have to expect that it will affect our health."