A young woman in her early 30s was the picture of health: She was fit and ran regularly but had this nagging cough that she couldn't seem to shake.
Because she didn't feel sick and had no fever, she figured it was a cold. Yet the coughing continued for weeks. So she went to her doctor, who suspected it was asthma because the hacking seemed to increase after she'd finished running. But asthma medications brought her no relief.
Frustrated that she couldn't run, the woman sought a second opinion from the Mayo Clinic in Rochester, Minn.
She learned the unexplained cough wasn't asthma. And it wasn't bronchitis or pneumonia either.
It was whooping cough.
In recent years, whooping cough has been diagnosed increasingly in teens and adults, although the disease typically had its most serious complications in infants.
Whooping cough is an illness caused by the bordetella pertussis bacteria. Health professionals tend to call this extremely contagious infection pertussis.
Reported cases of whooping cough have tripled in the United States since 2001, according to the Centers for Disease Control and Prevention, seen mainly in adolescents and adults, said Tami Skoff, a pertussis epidemiologist in the division of bacterial diseases at the CDC in Atlanta.
"But the jury is still out on whether it's a real increase in disease or an increase in reported disease due to improved diagnostic testing and more awareness among health professionals," she said.
Skoff suggested that there appears to be a shift in the epidemiology of the disease from infants to adolescents and adults. "From the data it looks like what's occurring is waning immunity."
A pertussis vaccine was first introduced in the United States in the 1940s, because whooping cough had been one of the most common diseases and a major cause of death in childhood.
The vaccine, known as DTaP, provides protection against pertussis along with diphtheria and tetanus. It's typically given as three shots at roughly 2, 4 and 6 months of age; a fourth shot is given between ages 15 and 18 months, and a fifth between ages 4 and 6, before children start school.
Although vaccination caused a sharp decline in the disease for more than 40 years, whooping cough never fully disappeared.
As more cases were reported, it became clear that the five-shot series was not providing lifelong protection. Immunity seemed to wear off five to 10 years after the last shot was received, making teens and adults vulnerable to infection once again.
In 2005, a new booster vaccine known as Tdap was approved by the FDA. This one-time booster dose is given to people between the ages of 11 and 64, assuming they have completed the childhood series.
But word of Tdap has not yet reached all parents of school-age children. While coverage rates for the childhood pertussis vaccine series hover around 90 percent of recommended recipients, rates for the Tdap booster are now around 30 percent, according to the CDC.
"It's still new," Skoff said. "With education campaigns, we expect this number to go up."