Every year in the United States, there are at least 20,000 confirmed cases of Lyme disease, a painful but treatable tick-borne illness. But now, experts are warning of a faster-acting, untreatable and potentially fatal sickness called Powassan virus that is most commonly found in the same tick that hosts Lyme disease.

Within two to three hours of being bitten by a tick infected with Powassan virus, a person may develop headaches, nausea, vomiting, muscle weakness, memory loss and speech difficulty. While people with Lyme disease develop lesions and a rash, those who get Powassan virus do not. In severe cases, the Powassan virus can infiltrate the central nervous system, causing inflammation in the brain and spine, and leading to encephalitis meningitis, said Dr. Theodore Andreadis, director of The Connecticut Agricultural Experiment.

“It’s (Powassan virus) not something pleasant,” Andreadis, whose organization is a Connecticut State-run agricultural research agency, told FoxNews.com. “We’re not putting any major alarm out there, but we’re saying we need to be aware of this because we do appear to be seeing a re-emergence of it.”

The virus is found in about 2 to 3 percent of the ixodes scapularis, the primary tick that hosts Lyme disease and the Powassan virus, while Lyme disease is found in 30 to 40 percent of them, Andreadis said. But unlike Lyme disease, which can take a couple of days to infect the person bitten after the tick latches onto his or her skin, the Powassan virus can transmit in a fraction of that time.

“The point being, if you find a tick feeding on you and you have the opportunity to get it off you within a few days, and it’s Lyme disease, you might be OK,” Andreadis told FoxNews.com. “With the Powassan virus, ticks will inject the virus within a matter of hours.”

Also, unlike Lyme disease and another tick-borne illness— such as the malaria-like Babesiosis, which stems from a microscopic protozoa called Babesia— Powassan virus cannot be treated by antibiotics. People who become infected with the virus have only supportive care available. It is thought to be fatal 10 percent of the time, Andreadis said.

Spotting these specific ticks can prove challenging, as their nymph stage is about the size of a poppyseed, according to the Centers for Disease Control and Prevention (CDC). The reason why Andreadis and his colleagues worry over a rise in cases this year is the virus’ favorite host, the white-tailed deer, are growing in population in Connecticut, where Andreadis’ team is based, and in the Northeast.

“With the increased forestation in the state and in the Northeast, the deer herd have increased exponentially, then the ticks have increased, and now the virus,” he said.

These specific tick-borne illnesses appear to crop up the most in the northeastern United States: In 2013, 95 percent of the confirmed Lyme disease cases were reported in this part of the country.

That year, the most recent data available, there were 12 confirmed human Powassan virus in the U.S., compared to seven in 2012 and 12 in 2011, according to the CDC. The Powassan virus was first identified in 1958, and ever since there have been 70 human cases reported, Andreadis said.

Durland Fish, professor of epidemiology at Yale University, who has spent his career studying Powassan virus and other tick-borne illnesses, said the number of actual human Powassan viruses is likely higher. That’s because many public health agencies haven’t developed tests for the virus and may mistake it as Lyme disease, or not spot the virus in people who may have both Lyme disease and Powassan virus. He added that in some regions of the U.S., the virus has been found in anywhere from 5 to 10 percent of ixodes scapularis ticks.

“We’re seeing some [Powassan] cases and some deaths, but not in proportion to what we’d expect to see considering how many ticks are infected,” Fish told FoxNews.com. “These ticks transmit five other kinds of diseases that we know about, so here’s another one. We know people are getting Lyme disease and Babesiosis because there’s tests for them, so there’s no reason to expect people aren’t getting this virus.”

Andreadis advised hikers and campers, or anyone going into a wooded area with low-lying brush— conditions that support tick populations— to wear long pants and tuck them into their socks, which can make spotting ticks easier.

“Examine yourself closely when you leave the area because these ticks will attach and begin to feed very quickly,” he said. “They like to attach around the waist, behind your knees, and even below your hairline. You want to check yourself quite closely.”