For some unlucky people, time in the great outdoors leads to Lyme disease, an illness causing head, joint and muscle pain, flu-like symptoms, fatigue, and sometimes a rash. Left untreated, those effects can turn debilitating and expand to include paralysis, inflammation of the brain and heart, and problems with memory, hearing and vision that can last for years.

Lyme disease, expected to affect more than 600,000 people in the U.S. this year, is caused by a spiral bacterium that spreads to humans through bites from infected deer ticks. The microbe provokes a complicated immune reaction that can resemble the response to other dangerous tick-borne bacteria, often making it harder for doctors to initiate the correct antibiotic treatment.