The sudden death last week of former U.S. Sen. Kay Hagan to a rare illness had people wondering how she contracted encephalitis. North Carolina’s first and only Democratic female U.S. senator died Oct. 28 at her Greensboro home. She was 66. Hagan died in her sleep after a three-year battle with encephalitis, caused by tick-borne Powassan virus. The infection causes irreversible inflammation of the brain.

In 2008, the former state legislator defeated Republican Sen. Elizabeth Dole. She lost to Republican Thom Tillis in 2014. “Susan and I are absolutely heartbroken by Sen. Kay Hagan’s sudden passing,” Tillis said in a statement, “and we extend our condolences and prayers to her loving family and many friends. We join all North Carolinians in remembering her dedicated and distinguished record of public service to our state and nation.”

In late 2016, Hagan became ill. Symptoms of severe Powassan encephalitis include confusion, loss of coordination, difficulty speaking and seizures. Her husband, Chip Hagan, said he initially thought his wife was having a stroke. Later, he said he suspected she was bitten by a tick during a Thanksgiving trip to the Blue Ridge Mountains. Two weeks later, she was rushed to a hospital. Symptoms of a tick bite may not reveal themselves for a week or more.

Doctors eventually determined Hagan suffered from Powassan virus. It’s less common than other tick-borne diseases such as Rocky Mountain Spotted Fever and Lyme disease. North Carolina regularly ranks as one of the states with the most reported tick-borne cases in the United States each year.

Sen. Hagan had difficulty controlling her muscles. She was confined to a wheelchair and rarely made public appearances. The number of reported cases of people sick from Powassan virus has increased in recent years. Most cases in the United States occur in the northeast and Great Lakes region from late spring through midfall when ticks are most active.

There are no vaccines to prevent or medicines to treat the disease. The likelihood of infections is increasing, according to data compiled by the Centers for Disease Control and Prevention. In 2017, local and state health departments reported a record number of cases. Lyme disease, Rocky Mountain spotted fever and other tick-related illnesses increased from 48,610 in 2016 to 59,349 in 2017. Reported tick-borne illnesses in the United States doubled between 2004 and 2016.

Ticks become infected when they feed on groundhogs, squirrels, mice or deer that have the virus in their blood. Infected ticks then spread the virus to people and other animals by biting them. Three types of ticks spread Powassan virus and are primarily found in the eastern half of the United States: the groundhog tick, squirrel tick and blacklegged or deer tick. The blacklegged tick was the culprit in Sen. Hagan’s case.

Blacklegged ticks are aggressive. They often bite people. Ticks can attach to any part of the body but are often found in hard-to-see areas such as the groin, armpits and scalp. In the absence of a vaccine, prevention of Powassan virus disease depends on measures to decrease exposure to infected ticks. These include avoiding wooded and bushy areas with high grass and using repellents to discourage tick attachment.