Lyme disease is a bacterial infection that features a skin rash, swollen joints and flu-like symptoms. You get the disease from the bite of an infected tick. Sometimes it is hard to know if you have Lyme disease because you may not have noticed a tick bite.
In the early stages, doctors look at your symptoms and medical history to figure out whether you have Lyme disease. In the later stages of the disease, lab tests can confirm whether you have it.
MedlinePlus - Lyme Disease