Every day for the past few weeks, John Aucott has diagnosed at least one new patient with Lyme disease.

It is prime season for the tick-borne disease in many parts of the East Coast, including Baltimore, where Dr. Aucott is director of the new Lyme Disease Clinical Research Center at Johns Hopkins University School of Medicine’s division of rheumatology.

For most people, Lyme disease, when caught early enough, is resolved after two to four weeks of treatment with antibiotics. But for as much as an estimated 10% of patients treated for the disease, symptoms, such as severe muscle and joint pain, fatigue and cognitive difficulties, can last for months or even years. The condition is called post-treatment Lyme disease syndrome, or PTLDS, and experts are divided on what it is, what causes it and how best to treat it.

“We’re at the primitive stage with that group of patients [with PTLDS] because we don’t have accurate biomarkers or blood markers to gauge what’s driving the disease. And there’s no FDA-approved therapy for those patients,” Dr. Aucott said. For the Lyme disease research center, which opened in April, “the mission is really to focus on the complex issue of chronic Lyme disease.”

Lyme disease is the most common tick-borne disease in the U.S. There are an estimated 300,000 new cases a year, about triple the rate from two decades ago, said Paul Mead, chief of epidemiology and surveillance activity for the Centers for Disease Control and Prevention’s Lyme-disease program. Most cases are centered in the Northeast, mid-Atlantic region and some north-central states, including Minnesota and Wisconsin. It is transmitted by the blacklegged tick, also known as the deer tick. On the West Coast, where it is less common, it is spread by the western blacklegged tick. The disease was named after a cluster of cases that broke out in Lyme, Conn., in the 1970s.