Kids who carry whooping cough can put their younger siblings in danger.

RODALE NEWS, EMMAUS, PA—As children head back to school, parents in school districts across the country are receiving letters about a disease they probably haven't thought much about since their children were infants. Instances of pertussis, or whooping cough, are on the rise this year, and some schools are requiring that children get booster shots for the whooping cough vaccine to protect vulnerable children.

THE DETAILS: Whooping cough is a very severe bacterial infection, and vaccination is the only known prevention. The coughs, which usually start out mild, become so severe that children emit the characteristic "whoop" as they suck in air after coughing. The disease is particularly fatal for infants, who often fall ill after being exposed to older siblings or adults who have unknowingly been carrying the disease.

Rates of whooping cough surge every three to five years, with the most recent spike occurring in 2005, says Harry Keyserling, MD, professor of pediatrics at Emory School of Medicine and a member of the committee on infectious diseases at the American Academy of Pediatrics. "So it would not be unusual to have a peak about this time," he says. According to the Centers for Disease Control and Prevention (CDC), 25,616 cases of pertussis were reported in 2005, and approximately 13,000 were reported in 2008. As of July of this year, the agency had tracked 6,408 cases, but pertussis season still has a few more months to go before it peters out in October.

However, some states are seeing even larger spikes in pertussis cases. California's department of public health declared an epidemic at the end of June after the number of whooping cough cases increased fivefold over the same period last year. Seven infant cases in the state have been fatal already. Other states, including South Carolina, New York (upstate), and Michigan, have seen unusual increases as well, although none matches the levels California is seeing.

However, says Dr. Keyserling, there's a difference between reported cases and actual cases. "The tests we use to make a diagnosis are not very accurate," he says, "and by the time someone presents with a chronic cough, the bacteria responsible for inducing cough are not present." That means that there could be more than a million cases of undiagnosed pertussis that are never reported to the CDC.