Lyme disease is the most commonly reported vector-borne illness in the United States, according to the Centers for Disease Control and Prevention (CDC).  

A vector-borne illness is one which is spread to humans through a vector, such as an insect or arachnid, carrying the disease.  In the case of Lyme disease, the vector is a tick.  More precisely, the illness is spread through the bite of a black-legged tick, or deer tick.