Dec. 4: Eleven workers at Quality Pork Processors in Austin, Minn., are diagnosed with a mysterious neurological condition, which causes weakness and shaking in their limbs. Minnesota Health Department announces investigation. The state is joined by the U.S. Centers for Disease Control and the CDC's National Institute for Occupational Safety and Health.

Dec. 11: One more worker is identified with symptoms of the illness, bringing the total to 12. Quality Pork Processors stops using a compressed-air system to remove brains from pig heads because affected employees worked at or near the area where it was used. The plant provides employees with more protective clothing and face masks.

Dec. 14: Health officials and Mayo Clinic doctors investigate whether illness is caused by an infectious agent, a toxin, exposure to animal tissue or an unknown factor.

Jan. 17: Two workers at an Indiana pork processing plant develop symptoms of the illness, the first cases reported beyond the Austin plant.

Jan. 31: Investigators focus on pig brain tissue, liquefied and sprayed into the air by the compression system, where they believe it was inhaled by workers -- possibly triggering an immune system response.