What Causes Lyme Disease?

I tested positive for lyme disease. But my only symptom is arthritis in my knees, which I have had for years. I don t see a time I could have been bitten by a tick in at least two months, more likely 3 months, and even then my last time in the woods was in early winter weather - temp maybe in the 40s, no sun. *THat was Dec 7 and today is March 7). Is it possible to have a mild case? I have never had a target rash and have not found a tick on me in years. I will start a round of antibiotics in any case, but I wonder - is it possible to have a mild case without severe symptoms? Tim Frazer Concord NH YYYY@YYYY