Whooping cough is caused by a bacterium called Bordetella pertussis, which infects the lining of the airways and triggers intense, uncontrollable coughing fits. The bacteria spread through the air when an infected person coughs or sneezes, releasing tiny particles that others breathe in. Close, prolonged contact makes transmission especially easy, and some people carry and spread the bacteria without realizing they’re sick.
How the Bacteria Attack Your Airways
Bordetella pertussis is a tiny, rod-shaped bacterium that targets the cells lining your respiratory tract. Once inhaled, it attaches to the hair-like structures called cilia that normally sweep mucus and debris out of your lungs. The bacteria produce several toxins that paralyze these cilia and damage the airway lining, essentially disabling your body’s natural cleaning system.
One key toxin punches holes in the membranes of immune cells, flooding them with a signaling molecule that disrupts their normal function. Another toxin interferes with your immune system’s ability to detect and fight the infection in its early stages. The combined effect is a buildup of thick mucus in your airways that your body can only clear through violent, repeated coughing. That coughing itself becomes the hallmark of the disease.
How Whooping Cough Spreads
The bacteria travel in respiratory droplets. Coughing, sneezing, or even sharing breathing space for extended periods can pass the infection from one person to another. Households, classrooms, and daycare settings are common sites of transmission because people spend long stretches in close proximity.
What makes pertussis particularly hard to contain is that it’s most contagious in its earliest stage, when symptoms look like nothing more than a mild cold. People with partial immunity from an old vaccination may develop only a lingering cough, never suspecting they have whooping cough, yet still spread the bacteria to those around them.
The Three Stages of Infection
Whooping cough progresses through three distinct phases, and the total illness can last weeks to months.
Stage 1: The Cold-Like Phase
The first stage looks like an ordinary upper respiratory infection: runny nose, low-grade fever, and a mild cough that gradually worsens over one to two weeks. This stage is when the bacteria are multiplying rapidly and when you’re most likely to pass the infection to others. In young infants, this phase may include episodes of pausing breathing (apnea) rather than obvious coughing.
Stage 2: The Coughing Fits
This is the stage that gives the disease its name. Coughing comes in rapid, uncontrollable bursts, sometimes 15 or more episodes in a 24-hour period. At the end of a coughing fit, the person gasps for air, producing the characteristic high-pitched “whoop.” These attacks frequently cause vomiting, exhaustion, and sometimes a bluish tint to the skin from lack of oxygen. They tend to be worse at night. The fits increase in frequency over the first one to two weeks, plateau for two to three weeks, then slowly taper off.
Stage 3: Recovery
The coughing fits gradually become less severe and less frequent over two to three weeks. But full recovery is slow. Coughing episodes can recur for months afterward, often triggered by subsequent colds or other respiratory infections. In China, pertussis has historically been called the “hundred-day cough” for this reason.
Why Infants Face the Greatest Danger
Young infants, especially those under two months old, are the most vulnerable to serious complications. Between 2000 and 2017, 307 people in the United States died from pertussis, and 84% of those deaths were in babies younger than two months. During that same period, pneumonia occurred in nearly 19% of infants under six months with pertussis, compared to about 13% of all reported cases.
Infants are also more prone to neurological complications, including seizures and brain damage from oxygen deprivation during prolonged coughing fits. Babies this young haven’t yet received enough vaccine doses to build meaningful protection, which is why the timing gap between birth and the first vaccinations represents a critical window of vulnerability.
Why Vaccination Doesn’t Eliminate the Disease
Pertussis vaccines work, but their protection fades over time. In the first one to three years after vaccination, effectiveness sits around 80 to 84%. By four to seven years out, it drops to roughly 62%. At eight or more years since the last dose, protection falls to about 41%. For each year that passes after vaccination, the odds of testing positive for pertussis rise by approximately 27%.
This waning immunity helps explain why whooping cough continues to circulate even in countries with high vaccination rates. Older children and adults whose protection has faded can become infected with milder symptoms and unknowingly pass the bacteria to unvaccinated or partially vaccinated infants. The disease follows a cyclical pattern, with periodic surges. In 2024, reported cases in the United States increased to more than six times the number seen in 2023, returning to pre-pandemic patterns of over 10,000 cases per year.
Protecting Newborns Before They Can Be Vaccinated
Because newborns can’t complete their own vaccine series for months, the primary strategy for protecting them starts during pregnancy. Getting vaccinated between 27 and 36 weeks of pregnancy (ideally in the earlier part of that window) allows the parent’s body to produce antibodies that cross the placenta and give the baby temporary protection at birth. This approach prevents 78% of pertussis cases in infants under two months and is 91% effective at preventing cases severe enough to require hospitalization.
A second layer of protection, sometimes called “cocooning,” involves making sure everyone who will be in close contact with the newborn is up to date on their own pertussis vaccination, ideally at least two weeks before meeting the baby. Cocooning alone isn’t always sufficient, since it’s difficult to ensure every person around an infant has been vaccinated, but it adds meaningful protection when combined with vaccination during pregnancy.
How Whooping Cough Is Diagnosed
Diagnosis depends on timing. In the first two weeks after the cough begins, a bacterial culture from a nasal swab can identify the organism directly. A more sensitive DNA-based test (PCR) can detect the bacteria up to three to four weeks after cough onset. Beyond that window, testing becomes unreliable because the bacteria may no longer be present even though the coughing continues. At that point, diagnosis often relies on symptoms and known exposure to a confirmed case.
Early diagnosis matters because antibiotic treatment is most effective when started during the first stage, before the severe coughing fits begin. Once the coughing fits are established, antibiotics can reduce how long you’re contagious but do little to shorten the course of the illness itself. The damage to the airway lining has already been done, and the cough will resolve only as those tissues heal.