This year’s crop of worries includes West Nile virus and the potentially deadly eastern equine encephalitis, or EEE, along with dengue spreading from Puerto Rico to Southern states.

So far in the U.S. this year, 26 people have died from West Nile and at least one from EEE, according to preliminary data through early September from the Centers for Disease Control and Prevention.

Public-health officials and experts note that serious health complications and deaths from either virus are rare; most people have no symptoms or mild ones. Steps you can take to protect yourself include staying inside at dawn and dusk when mosquitoes are especially active, and wearing insect repellent.

A growing problem

Vector-borne diseases—those transmitted by blood-feeding creatures like mosquitoes and ticks—are “among the fastest-growing group of diseases right now on the planet,” says Dr. Lyle Petersen, director of CDC’s division of vector-borne diseases.

Climate change and rising temperatures are one reason, says Petersen. And it isn’t just because the first frost might come later. “When the temperature is warmer, the virus multiplies faster in a mosquito and it multiplies in higher numbers,” he says. “So mosquitoes become infectious faster—and more infectious.”

In the U.S., West Nile has been reported across the country, with more than 375 cases reported so far, according to the most recent CDC data, including 255 that are neuroinvasive, meaning the virus entered the central nervous system.

So far this year, there have been six EEE cases reported, but the virus has been detected at higher-than-normal levels in tested mosquitoes, mainly in the Northeast, says Laura Harrington, a professor of entomology at Cornell University who studies human- and animal-mosquito interactions.

Meanwhile, a dengue outbreak in South America and the Caribbean is spilling over into Florida and elsewhere.

Mosquitoes are attracted to odors and the carbon dioxide we breathe out, says Stephen Rich, professor of microbiology and director of the New England Center of Excellence in Vector-Borne Diseases at University of Massachusetts Amherst. It is unclear if they are more attracted to certain people, but people’s sensitivity to the bites can vary.

West Nile

West Nile virus cases happen every year. Those hit hardest are usually the very young and old, and people with compromised immune systems, says Rich.

In the U.S., about 1,200 people on average are diagnosed with neuroinvasive disease from West Nile virus every year, and more than 100 die from the virus, says Dr. Erin Staples, a medical epidemiologist with the CDC’s division of vector-borne diseases.

Most people with West Nile don’t develop symptoms, she adds. About 20% of people will experience mild symptoms, such as fever, headache and joint pains. For a small proportion of people, the virus enters the central nervous system and can cause swelling or inflammation in the brain, resulting in encephalitis, meningitis and other serious conditions, says Staples.

West Nile cases are in every state, though there tends to be a higher incidence in the Great Plains and Southeast states, says Staples. It typically circulates in fresh hardwood swamps.

Eastern equine encephalitis

EEE cases are far more infrequent, totaling about half a dozen reported U.S. cases, or double digits in a big year.

EEE can infect anyone, says Rich, but about 95% of the people who are bitten by a mosquito infected with the virus have no symptoms. Of the remaining 5%, about a third will develop encephalitis, a life-threatening complication. The other two-thirds have mild symptoms, such as fever, headache or achy joints.

The virus is concentrated in the Northeast. Several Massachusetts towns have shut down parks in the evening recently because of worries about infection. “Not being outside when mosquitoes are most active—which is dawn and dusk—probably does reduce the risk of exposures,” he says.

Dengue

Dengue is transmitted by Aedes mosquitoes, a different type than the one that spreads EEE and West Nile. Aedes mosquitoes are more common in the Caribbean as well as Central and South America, but the U.S. has them in some areas, including Florida.

Large dengue outbreaks overseas raise the risk of dengue in the U.S. Infected travelers return to the U.S. and Aedes mosquitoes in this country can pick up the disease from those humans.

More than 4,100 cases have been reported in the U.S. this year, according to the most recent CDC data, counting people who were infected both inside and outside of the country.

About 25% of people infected with dengue get sick. Symptoms, which can be mild or severe, can include fever, aches and pains, nausea, vomiting and rash. Most people recover within a week.

About one in 20 people with dengue will develop a severe case, which can be deadly. Warning signs include bleeding from the nose or gums, vomiting at least three times in a day, belly pain and tenderness, and feeling extremely tired.

How to protect yourself

Avoid the outdoors during dawn and dusk, and apply insect repellent with DEET or picaridin, experts suggest. Wearing long sleeves, pants and socks is also a good idea.

If you work outside often, you can treat clothes with permethrin, which kills ticks and mosquitoes. Make sure to use an insect repellent registered with the Environmental Protection Agency.

Regularly draining standing water in your yard, installing screens on your windows and doors, and using air conditioning rather than opening windows are other strategies that can help.