So why is Down Syndrome still a "syndrome?" It's a Trisomy 21 disorder, causes measurable anatomical effects and has a fairly consistent set of symptoms, that vary with the severity of the disorder.
To me, Lyme Disease is more of a syndrome by that definition. Yes, the cause is known, but the symptoms can be nonexistent or life threatening, and tend to change. Some people may have fever and joint pain. Others may not have either, but may just have fatigue and headaches. The symptoms aren't that consistent, or there wouldn't be so many people who go undiagnosed for so long, whereas most doctors can spot a Down Syndrome baby at birth. So many people who have Lyme Disease are "finally" diagnosed with it after being treated for a dozen other disorders first.