If Lyme disease is so easily treated, why do so many people report chronic symptoms?

Dr. Shapiro said a small number of people may be especially sensitive to the lingering effects of the Lyme infection, even after the bacteria are long gone. But he said most people who report chronic problems were most likely wrongly diagnosed with Lyme disease when they had something else to begin with.

There is a blood test for Lyme disease, but its use and interpretation require some sophistication. The blood test detects antibodies the body makes against Lyme bacteria, but these antibodies are not detectable until about four weeks after the bite — and two to three weeks after the rash typically appears. So if doctors order the test when they see the rash, the test would deliver a “false negative” even when there is an infection.

The test picks up the antibodies if the person has ever before been exposed to the bacteria — which, in areas where Lyme is prevalent, is not uncommon. The Lyme test is also not specific and can react to other antibodies in the body. Either of these scenarios can deliver a “false positive” test result, suggesting an active infection where there is none.

For these reasons, the test has to be used in conjunction with symptoms in order to make a clear diagnosis, Dr. Shapiro said.

Some people who have unexplained pain or fatigue may latch on to Lyme disease as a possible explanation after getting an inaccurate diagnosis from a doctor who misinterprets the test. The doctor may then treat them for Lyme disease. “And guess what? They’re not cured — because it wasn’t Lyme disease in the first place,” Dr. Shapiro said.

Those patients may continue to take antibiotics for months in the hopes of getting better, but that long-term treatment is harmful , he said. “It’s costly, and people don’t get treated for what they really have.”

In places where Lyme disease is prevalent, pediatricians often diagnose based on the symptoms alone and use the test for confirmation.