First, the species brings diseases from its original location. Second, the ticks pick up new pathogens from animals in their new ecosystem.

Physicians and patients in a tick’s new home may be less familiar with the diseases it carries. They can overlook symptoms or attribute them to a different cause, delaying effective treatment.

The best known threat is Lyme disease. Cases in the United States increased from about 12,000 annually in 1995 to nearly 40,000 in 2015. Experts say the real number of infections is likely closer to 300,000.

But scientists are finding ticks carry more than just Lyme: At least a third of known tick-borne pathogens were found in the last 20 years. Heartland virus and Bourbon virus, which can prove fatal, were discovered in just the last five years.

Powassan virus, a rare but dangerous pathogen that can cause permanent brain damage or death, can be passed from tick to human in just 15 minutes. It was discovered in 1958, and an average of seven cases are reported each year. Earlier this month, a resident of Saratoga County, N.Y., who had Powassan disease died.

Dr. Gary Wormser, founder of the Lyme Disease Diagnostic Center at New York Medical College, said the most worrisome tick-borne contagion he sees is babesiosis, which can cause malaria-like symptoms and require hospitalization. A few of his patients have died from it; several required intensive care.