The hallmarks of a warming climate, heavier rains, more severe droughts, rising sea levels and longer growing seasons, are spreading a variety of pathogens throughout the world. Malaria is moving to the highlands. Lyme disease is spreading across the U.S. Northeast and eastern Canada. Outbreaks of cholera will increase with more unsafe water.
Those are three of the diseases that are becoming part of a growth field in medical research amid concerns that tropical diseases are moving north and south and that progress made to improve health conditions in previous decades might be undone.
"I'm quite worried that many of the gains we've made with the [anti-poverty] Millennium Development Goals as far as reducing major infectious diseases and childhood mortality ... could be reversed and lose quite a lot of ground because of climate change," said Jonathan Patz, director of the Global Health Institute at the University of Wisconsin, Madison.
Many different factors affect whether and how much an infectious disease spreads, including the level of development in a country, the public health and sanitary infrastructure, and how safe and plentiful drinking water is.
But scientists and public health groups are now focusing more attention on how shifts in the climate may accelerate problems. One example is the negative health impacts from heavy flooding that results from rising sea levels and more frequent storms.
"We have several studies around the world that scientifically link these extreme precipitation events to outbreaks," said Joan Rose, a water expert at Michigan State University, with diseases like hepatitis A, giardiasis and norovirus infection occurring because of contamination to drinking water.
Signs of movement
One study at the Johns Hopkins Bloomberg School of Public Health found that from 1948 to 1994, 51 percent of 548 reported waterborne disease outbreaks in the United States were preceded by extreme precipitation. The insurance company Munich Re has said that weather-related natural disasters increased nearly fivefold in North America over the last three decades.
One example is ciguatera, an illness that people get by eating fish that have a certain type of algae toxin; the disease causes nausea and vomiting and has neurological effects.
"We never saw them outside the Caribbean ... but now [we've] started seeing these cases in the northern Gulf of Mexico, because we have increased sea temperatures to allow the organism to survive there," said George Luber, associate director for climate change at the Centers for Disease Control and Prevention. "Critically, there's also another influence of the creation of artificial habitat -- oil rig platforms and artificial reefs."
Malaria, a mosquito-borne infection caused by a parasite, is one often-cited example of how a disease can be affected by climate change, although a few scientists disagree about how strong the link is. Malaria, which can cause low red blood cell counts and cut off vessels carrying blood to the brain, caused an estimated 627,000 deaths in 2012, with many of its victims children in Africa.
"It's very well-documented the climate sensitivity of malaria, so there have been studies showing that malaria will likely move up in altitude, for example, in the east African highlands, with projected warmer temperatures," said Patz. An area that's too cold doesn't let malaria develop.
Referring to mosquitoes carrying dengue virus, Patz said there's evidence that shows warmer temperatures in water that contain mosquito larvae lead to faster development and smaller mosquitoes that need to bite people more to develop their eggs.
"When you have smaller mosquitoes carrying dengue fever, you can increase the biting rates, as well, just by having warmer water temperatures," he said.
Closer to home, in the lower 48 states plus the District of Columbia in 2013, there were 2,474 documented cases of West Nile virus, characterized by symptoms like headache, high fever and joint pains; it led to 114 deaths last year, according to the CDC. First detected in New York in 1999, it quickly spread across the country. But researchers say the disease hasn't been in the United States long enough for them to make a definitive climate link. A recent study found that West Nile has cost the United States $800 million since it arrived in 1999.
Drought and longer growing seasons
"What we see in that work is absolutely a longer season for the relevant mosquitoes to survive in the U.S.," said Andrew Comrie, a professor at the School of Geography and Development at the University of Arizona. "You find the season lengthening by as much as two to three weeks ... and therefore, [that] exposes people earlier and later in the season."
Droughts can make a disease like West Nile worse, given that it thrives in still puddles of water found in places like the bottoms of storm drains. "In the summertime, if you don't get big rainfall events that flush out the storm drainage where mosquitoes can be breeding, in that condition, the drought can worsen things," Patz said.
Tick-borne Lyme disease has also been steadily increasing over the last decade. There's been "a continual range expansion of the tick" that has led to more cases, said Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies. One key reason that set the stage decades ago is the reforestation of much of New England, which makes it much easier for ticks and their hosts, like mice and deer, to thrive.
But "climate change is almost certainly having an impact on the northward and westward expansion into more extreme climates, [and] those places are becoming increasingly hospitable to tick populations, places that were probably too cold before," he said.
For example, New Hampshire saw 887 reported cases of Lyme disease in 2011, up from 190 in 2002, according to CDC data.
Alan Eaton, an entomologist at the University of New Hampshire, said that during the time that Lyme has been surging in the state, the periods of snow cover in the winter has declined, there's been an increase in average winter temperature, and there have been some changes in rainfall, as well. Those factors could be supporting the growth of ticks in the state, although Eaton cautioned that "you can't prove cause and effect" yet.
"Brain-eating amoeba" sound like something a zombie would infect you with. But the pathogen that causes the disease, Naegleria fowleri, is actually a microscopic organism that thrives in warm freshwater lakes and is almost always fatal. Although it's still rare, recent cases have occurred in Florida, Arkansas and Louisiana.
"As freshwater lakes get hotter in the summer, that leads to more amoebae in the water and increased human risk," said Sonia Altizer, an associate professor of ecology at the University of Georgia.
The climate-linked diseases are not just affecting people; they're also hitting plants and animals.
Risks to coffee and butterflies
For example, coffee-leaf rust, or Hemileia vastatrix, is a fungus that physically weakens coffee plants, prevents them from growing properly and ruins the crops for farmers. It has spread across the world, and the International Coffee Organization said it cost growers in Central America $500 million and 374,000 jobs in the 2012-13 crop year.
One major reason the rust was so devastating in Central America was unusually heavy rainfall at high attitudes where coffee is grown. While single rain events can't be tied to climate change, heavier precipitation overall does have a climate link.
Migratory patterns can also change. Some elegant orange-and-black monarch butterflies are now staying in the southern United States during the winter months instead of migrating down to Mexico.
"What we're finding is that this actually increases the transmission of a debilitating protozoan that infects them," said Altizer. "When the animals migrate, the migration weeds out some of the infected butterflies that can't fly as well, and it also allows the butterflies to escape from their summer habitat, where ... parasites actually build up during the breeding season."