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.
Over the weekend, health officials contacted susceptible individuals who might have visited health facilities near the same time as the patients, to confirm that they had been vaccinated or to encourage them to get a shot now.
Since the first cases were identified, the department has vaccinated more than 100 people, and many more probably got doses at their private doctors, Dr. Jones said.
Those who have been immunized have little to worry about, but he said the number of infections was likely to rise.
“I will be shocked if we don’t have many more cases,” he said.
Until the recent outbreak, the state had confirmed just nine cases since 2004.
Despite the outbreak, there has been a significant decrease in measles patients so far this year in the United States.
As of April 1, four people from three states — California, Georgia and Texas — were reported to have measles, according to the Centers for Disease Control and Prevention.
By contrast, 159 cases from 18 states and the District of Columbia were reported in for the same time period in 2015. Health officials were worried then because 70 percent of those cases were linked to an outbreak that started in December 2014 in Disney theme parks in Orange County, Calif.
The infection flourished mostly among unvaccinated children and adults who did not know if they had been vaccinated. Thirty more cases were reported by the end of the year.
In 2014, the United States had the record number of measles cases — 667 — since the disease was thought to have been eliminated in 2000. Nearly 400 of them occurred in unvaccinated Amish communities in Ohio. At the time, the Philippines was having a major outbreak, and many of the cases in 2014 were imported from there.
Measles spreads through the air and is highly contagious. Not only does it pass through droplets in the air, but it can also linger. Infected people can also spread measles before a telltale rash of flat red spots develops.
The measles virus “literally is like a tracking device,” said Dr. Manisha Patel, a medical epidemiologist at the C.D.C. “It will find susceptible people.”
Travelers bring the measles virus to the United States and transmit it to people who are unvaccinated, but since 2000, it no longer originates here.
Most infants do not have their first measles-mumps–rubella shot, known as M.M.R., until 12 months, leaving them vulnerable to infection if not enough people who are old enough to get vaccinated do. Measles can be serious in children younger than 5, sometimes causing pneumonia, brain swelling and even death.
The idea is that enough people in a population — or herd — must be vaccinated to protect the vulnerable who cannot be vaccinated, because they are newborns or because they are being treated for cancer with radiation or drugs.
After the Disney outbreak, a push ensued to get the M.M.R. shot to unvaccinated children, but some parents defended their right to raise their children without immunizations.
In the 2014-15 school year, the median rate of M.M.R. adoption for kindergarten students in United States was 94 percent. Tennessee reported 95.1 percent.
The C.D.C. says that the vaccination is about 93 percent effective, and that two doses are 97 percent effective. It recommends children receive their second dose at ages 4 through 6 years.
“If you can get vaccinated, you should get vaccinated,” Dr. Patel said. “That’s the best way to protect not only yourself, but other people around you that can’t get the vaccine, and to protect these outbreaks from happening.”