Partly geography,,, Though one may be bitten by a tick, it may not transmit lyme, depending on where you are and how long the tick has been feeding(ie. Ticks that carry lyme aren't found in az; in ca, the tick ixodes pacifica does carry it). However, as dr. Baker describes, there are many more other infections one can get from ticks, with a single feeding resulting in the presence of more than 2 or 3 types of bugs.
Systemic symptoms. It is common for the area around a tick bite to be inflamed & rarely the bite area can become infected. But ticks transmit many serious infections, including lyme disease, ehrlichiosis, babesiosis & rocky mountain spotted fever. Lyme may have a bullseye rash around the bite, but not always. Systemic symptoms like fever, headache, muscle & joint pains are signs that you may have tick-borne illness.