Serologic testing for antibodies can distinguish between acute and chronic infection, and can tell you IF you have been infected with Lyme. It really cannot distinguish an active infection from a past infection. A Lyme PCR can be done, it detects the presence of DNA from the spirochete. It can be used for monitoring infection. It may be an adjunct, but it is not recommended for the diagnosis of the disease. Diagnosis is still done on the basis of serology. You can be re-infected with Lyme at any time. "Relapses" are often re-infections. There are also other tick-borne diseases which have only recently been identified, and I would bet there are a few more that have yet to be identified.
Are there people who have antibodies that are undetectable? Likely. There have been a few studies looking at the persistence of the bacteria after treatment with antibiotics. There is no evidence of that occurring. There is a recognized post-Lyme disease syndrome which consists of joint pain, headaches, fatigue etc. It resolves in nearly everyone within 1 year, and occurs in only a small percentage of people. People who have it long-term can have a persistent and permanent arthritis. Nobody understands why. Doxycycline is and interesting antibiotic in that it has and anti-inflammatory effect. It can help things feel better aside from it's bacteria killing properties.
The symptoms of Lyme are fairly non-specific. They can mimic a lot of other diseases.
