Mosquito and tick season is upon us once again, and with it a new roster of diseases to worry about. Zika may have faded from the headlines, but yellow fever (which is spread by the same mosquito as the Zika virus) and Powassan (which is spread by ticks, but like Zika also causes neurological symptoms) seem poised to take its place. 

Powassan has emerged in Maine, where three cases were recently confirmed. And, for the first time since the 1940s, yellow fever has made its way to Brazil, where more than 200 people have been killed by the virus in the past three months. 

How far and wide either of these diseases will spread—and how many human victims they will ultimately claim—is still anyone's guess. 

Scientists have been trying to develop ways to forecast infectious disease outbreaks the same way they forecast the weather, but it's tricky work. "It's really difficult to predict when or where an outbreak will occur," says Rebecca Eisen, Ph.D., a research biologist with the Centers for Disease Control and Prevention's National Center for Emerging and Zoonotic Infectious Diseases. Whether a given mosquito- or tick-borne disease strikes depends on a complicated mix of factors that can vary greatly from one locale to the next.

"The best, most important thing you can do is to avoid getting bitten in the first place," Eisen says. "And part of that is knowing what bugs are in your area, and knowing what pathogens they transmit." 

Here's a breakdown of the insect-borne diseases that are likely to make headlines this season, along with some tips for protecting yourself from the bug bites that transmit them. 

New Threats Emerge

Powassan is not the only emerging tick-borne illness that infectious-disease experts are keeping an eye on this season, but it is certainly the one to garner the most headlines so far. 

The virus is spread by the same tick that carries Lyme, but is not nearly as common: on average, there are seven reported cases for Powassan each year (and just three, all in Maine, so far in 2017), compared with about 300,000 for Lyme.

Still, despite its rarity, Powassan does have some features that make it frightening. It is transmitted very quickly. The infection can pass from tick to human in less than an hour (Lyme disease is believed to take 24-36 hours to accomplish the same feat).

And it can be deadly: Powassan causes fatal brain swelling in about 10 percent of cases, and permanent neurological damage (vision trouble, facial tics, blurred vision) about half the time.

There is no vaccine against Powassan, and no treatment for the disease. 

Some scientists have predicted an uptick in cases of Powassan and other tick-borne diseases this season, owing to warmer winters and earlier springs. But it remains to be seen when, where, or whether those larger outbreaks will emerge.

In the meantime, as Eisen says, the most important thing to do is prevent ticks from biting you in the first place. See our tips for keeping bugs away

An Old Scourge Returns

Yellow fever (which is spread by the same mosquito that spreads Zika and dengue) once ranked as the largest public health threat in the western hemisphere. At the start of the previous century, before the vaccine to prevent it had been developed, this virus periodically decimated cities from New Orleans to Philadelphia.

In most cases, yellow fever cause no symptoms, or triggers only mild flu-like ones. But 15 percent to 20 percent of those infected can develop high fevers, jaundice, and bleeding, and those more serious cases are often fatal. 

So it's understandable that the virus's re-emergence in Brazil earlier this year has public health officials deeply concerned. So far the outbreak there remains "sylvatic," which means it is spread mainly  between forest-dwelling mosquitoes and monkeys, with humans only occasional victims. But experts worry that this may change, because the affected regions are dangerously close to dense urban centers.  

According to Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases, it's "highly unlikely" that we will see a yellow fever outbreak in the continental U.S., though it's possible in Puerto Rico, the U.S. Virgin Island, and other U.S. territories. There may also be isolated cases in the Gulf Coast region and among people who have recently traveled to the tropics, as there were with Zika.  

For information on how to protect yourself abroad, see our primer on travel vaccines.  

The Usual Suspects Persist

While the emergence of something newfangled, or the return of something long-forgotten, can inspire fear, Eisen and others say that the biggest insect-borne threats by far will be the same this year as they have been every year for the past decade or so. 

West Nile virus first emerged in the U.S. in 1999. Since then, it has established itself in all 48 of the continental states, triggering outbreaks across the country every summer for the past two decades, and infecting an estimated 2,000-plus people in that time. 

The virus (which is spread by the Culex mosquito), causes no symptoms in most people, though about a fifth develop flu-like symptoms. Most people recover completely, but fatigue and weakness can persist for months after the initial infection.

Less than 1 percent of patients develop severe illness, with neurological symptoms that include brain swelling (encephalitis), high fevers, disorientation, meningitis, seizures, and paralysis. About 10 percent of those patients die; the rest take weeks to months to recover, and can suffer some permanent neurological damage. 

West Nile outbreaks are most common between June and December, and are influenced by a range of factors, including the weather, the mosquito population, the number of infected birds, and as always, human behavior. 

The best way to protect yourself is to properly apply an effective insect repellent.   

Lyme disease remains the single largest insect-borne disease threat in the continental U.S. Each year, the CDC logs some 30,000 new case reports, though the agency suspects that the actual number of human Lyme infections is actually 10 times higher. 

The disease is caused by a bacteria that is spread by the infamous deer tick. The vast majority of cases occur in just 14 states (see our map below). Early symptoms of Lyme infection can include a bull's-eye-like rash around the area of the bite, but many people experience a different rash pattern, or no rash at all. 

In fact, it can also take as long as thirty days after a tick bite for any symptoms to appear, and those symptoms can often be so mild as to be overlooked or mistaken for flu. In rare cases, Lyme can cause severe long-term neurological effects. 

The best way to protect yourself from Lyme Disease is to prevent tick bites from occurring at all. The good news is there are several proven ways to accomplish that

Guide to Mosquito and Tick Diseases

Where Most Cases Occur

Serious Side Effects

Symptoms appear


Common Symptoms