PHILADELPHIA — Pertussis is at its highest level nationally in a half-century.

But cases of pertussis, also known as whooping cough, often decline in late fall into early winter.

With 16 deaths nationwide this year — most of them infants no more than 3 months old — a decline that is more than a typical seasonal variation could be good news, as pertussis usually appears in waves several years apart. On the other hand, several states, mainly in the West, have been fighting multiyear outbreaks; Washington state’s reached levels even higher than before a vaccine arrived in the 1940s.

At that time, pertussis caused 5,000 to 10,000 deaths a year in the United States. The classic “whoop, whoop” sound of children gasping for air amid coughing spasms set parents on edge. The disease is so contagious that up to 90 percent of close contacts who don’t have immunity will become infected; a single sneeze can do it.

Sarah Long, a baby in 1945, remembers her mother talking about the time when all five children in the family came down with pertussis. “She did not change clothes for two weeks,” said Long, chief of infectious diseases at St. Christopher’s Hospital for Children in Philadelphia. She stayed in the room “to make sure we would all make it through coughing spells.”

A “whole cell” vaccine using a killed Bordetella pertussis bacterium was added to diphtheria and tetanus toxoids as combination immunizations in the late 1940s; within two decades, pertussis in the United States was almost gone.

“Pertussis is a weird kind of disease,” said Gary Emmett, chief of inpatient pediatrics at Thomas Jefferson University Hospital. It looks like a cold: runny nose, aches, low fever, and a mild cough. It often goes away in adults after a week or two — plenty of time to have infected children.

The cough may continue, Emmett said, and “could be mistaken for asthma.” Some cases then worsen, with vomiting and coughing fits that lead to exhaustion.

The youngest infants are unable to gasp for air with the classic “whoop,” but many briefly stop breathing, known as apnea. More than half are hospitalized; of those, 1 percent to 2 percent die, the CDC reports