Health officials in Tennessee have confirmed six cases of measles in the Memphis area since Thursday, surpassing in one weekend the nationwide total for all of 2016.

Until the recent outbreak, there had been four cases reported this year, a steep decline from 2014 and 2015. But health officials say the frequency and intensity of outbreaks are unpredictable, and they were racing to try to contain this one.

Dr. Tim Jones, the state epidemiologist for the Tennessee Department of Health, said in a telephone interview that none of the six patients had been immunized. They included infants who were too young to be immunized and at least one adult who chose to forgo immunization, all in Shelby County.

What concerns Dr. Jones is that the six people were divided into three unrelated clusters in different parts of town, and officials had been unable to piece together how their paths might have crossed.