When 21 slaughterhouse workers at an Austin, Minn., pork plant came down with a mysterious neurological disorder in 2007, it prompted a nationwide investigation into the unique autoimmune disease.

Now, nearly three years later, researchers are reporting that they’re beginning to piece together the puzzle. In a study published Monday in Lancet Neurology they confirmed an initial theory — that the workers had inhaled pig blood and brain tissue sprayed into the atmosphere by high-pressure air hoses.

But they also found that many other meatpackers who worked in the same area and inhaled the same foreign tissue did not get sick. Together, the findings raise a more profound question: What was the difference?

“That is the great unknown,” said Dr. Daniel Lachance, a Mayo Clinic neurologist who was one of the first to identify the disease and has studied it ever since. “There are many, many instances in medicine where people have exposure but they don’t get sick.”