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Weather and infectious disease have long been intertwined—and this has become painfully evident in the past few decades.

In 1999, West Nile virus, which originated in Uganda and is spread by mosquitoes, killed seven in New York City. In 2014, another mosquito-borne virus, called chikungunya (which means “bending over in pain” in the Makonde language of East Africa), afflicted 12 Florida residents.

In each case, the virus to blame had not previously been reported in the affected area. Extreme or shifting weather patterns helped expand the habitat of the mosquitoes that can carry those illnesses—and experts say that outbreaks are bound to become more common as the effects of climate change become more prevalent.

“A longer warm-weather season and changing rainfall patterns are allowing the insects that can transmit disease to humans to thrive for longer periods each year—and to simultaneously move into broader areas,” explains Kim Knowlton, senior scientist and deputy director of NRDC’s Science Center.

Knowlton and other experts say climate change means Americans need to learn more about diseases that are vector-borne, or transmitted by agents such as insects. Among the biggest threats—because we have neither effective vaccines nor guaranteed cures for them—are West Nile virus and dengue fever, both spread by mosquitoes, and Lyme disease, transmitted by deer ticks.

Tick transmission

When a deer tick bites a rodent infected by the Borrelia burgdorferi bacterium, that tick can then bite humans and infect them with Lyme disease. More persistent warm weather has expanded the active period and the geographical range of the deer tick, also called the blacklegged tick.

Lyme disease is now the most commonly reported vector-borne disease in the United States. It has been diagnosed in every state except Hawaii since first being reported in 1975 in the Lyme, Connecticut, area—though 95 percent of confirmed cases are concentrated in 14 states in the Northeast, Mid-Atlantic, and upper Midwest.

A 2015 study by the Centers for Disease Control and Prevention found that 260 high-risk counties in the Northeast and upper Midwest had twice as many Lyme cases as authorities had suspected. Another study identified more than 400 counties—mostly in the upper Midwest and along the Atlantic and Gulf Coasts—as newly suitable habitats for one species of Lyme-carrying tick. The disease is also spreading to Canada.

Dark brown and with larvae as small as poppy seeds, deer ticks are difficult to spot. But if an infected tick clings to your skin and feeds on your blood for 36 to 48 hours, you may develop the bull’s-eye rash, flulike symptoms, and joint pain that signal infection. “If you’re infected with Lyme disease and don’t get treated right away, there can be long-term effects such as debilitating and persistent fatigue,” says Knowlton.

Mosquito matters

Parts of the United States that once had steady precipitation are now experiencing periods of drought punctuated by heavy rainfall. Sudden deluges leave behind puddles of standing water—moisture that mosquitoes need to hatch their eggs. In addition, hotter weather is shortening these eggs’ incubation times, increasing the overall mosquito population. Female mosquitoes are the ones that bite, and warmer weather makes them more likely to do so.

West Nile, now the most common mosquito-borne illness in the United States, is spread when mosquitoes bite infected birds and then go on to bite humans. While about four out of five people infected with West Nile do not develop symptoms, the virus occasionally causes fevers accompanied by body aches, disorientation, diarrhea, neck stiffness, headache, joint pain, and tremors. It can also spread to the brain, though fewer than 1 percent of people with West Nile will develop potentially fatal encephalitis or meningitis.

Dengue fever has long persisted in Asia and Latin America but recently spread to the United States as well. The four strains of the virus that cause it can bring on high fever, headache, pain behind the eyes, and crippling bone, muscle, or joint aches. “Most cases usually just require bed rest and lots of fluids,” says Knowlton. But contracting dengue also puts you at risk for a later bout with dengue hemorrhagic fever if you are infected a second time. This potentially fatal condition can cause bleeding from the nose or gums, easy bruising, and blood in vomit or stool.

Avoiding risk

Over the past 40 years, temperatures in the United States have risen an average of 0.29 to 0.46 degrees Fahrenheit per decade. Because climate change is now a fact of life, it’s essential that Americans protect themselves from rapidly spreading vector-borne diseases that have no definitive cures. If you find yourself in an area where ticks and mosquitoes are prevalent, Knowlton and other experts recommend that you:

Install or fix window and door screens so mosquitoes can’t enter your home.

Cover up when possible by wearing long-sleeve shirts and long pants.

Drain standing water from birdbaths, kiddie pools, storm-drain catchments, and other places where it can collect.

Spray insect repellent containing a low concentration of DEET (30 percent or less) on your clothes and exposed skin—but not your face—before stepping outdoors.

Ward off deer ticks during a hike by tucking pant cuffs into long socks.

Wear light-colored outdoor clothing so you can better spot dark deer ticks.

Check yourself for ticks when you return from outdoors.

Remove an attached tick using fine-tipped tweezers, grasping the tick as close to your skin's surface as possible. Then clean your skin and hands with rubbing alcohol, an iodine scrub, or soap and water.

In the shower, use a washcloth or loofah from head to toe to brush away any unattached ticks that may be on your body.

If you develop suspicious symptoms, seek immediate treatment to safeguard your health—and that of others in your area.