Pertussis, commonly known as whooping cough, is a highly contagious bacterial infection of the respiratory tract caused by the bacterium Bordetella pertussis. It gets its nickname from the distinctive high-pitched “whoop” sound people make when gasping for air after a violent coughing fit. While pertussis can affect anyone, it is most dangerous for infants under one year old, roughly one-third of whom require hospitalization.
How Pertussis Affects Your Airways
The bacteria attach to the ciliated cells lining your airways. Cilia are the tiny, hair-like structures that sweep mucus and debris out of your lungs. Once attached, the bacteria paralyze these cilia (a process called ciliostasis) and damage the tight junctions between airway cells. With the cilia no longer doing their job, mucus builds up and your body’s only remaining clearance mechanism is coughing, sometimes violently and uncontrollably.
The Three Stages of Illness
Pertussis unfolds in three distinct stages, and the total illness can drag on for weeks or even months. This is why it’s sometimes called the “100-day cough.”
Stage 1: Catarrhal (1 to 2 Weeks)
The illness starts with symptoms that look like a common cold: a runny nose, low-grade fever, and a mild, occasional cough. The cough gradually worsens over the course of one to two weeks. This stage is the most contagious and the hardest to diagnose because nothing yet distinguishes it from a regular upper respiratory infection. In infants, pauses in breathing (apnea) may appear even at this early stage.
Stage 2: Paroxysmal (2 to 6 Weeks)
This is the stage most people associate with whooping cough. Coughing comes in intense, rapid-fire bursts called paroxysms, often followed by the characteristic whooping sound as you struggle to inhale. These fits can cause vomiting, exhaustion, and even a bluish tint to the skin from lack of oxygen. On average, patients experience about 15 coughing attacks per 24 hours, and they’re typically worse at night. The attacks increase in frequency over the first one to two weeks, plateau for two to three weeks, then slowly decrease.
Stage 3: Convalescent (Weeks to Months)
Recovery is gradual. The paroxysmal coughing becomes less frequent and usually disappears within two to three weeks of entering this stage. However, coughing fits often return temporarily whenever you catch another respiratory infection, and this pattern can persist for many months.
Pertussis in Infants
Babies under three months old face the greatest risk. In very young infants, the classic whooping cough presentation may not appear at all. Instead, apnea (pauses in breathing) may be the primary symptom, sometimes with gagging or gasping but minimal coughing. This atypical presentation makes the disease easy to miss and especially dangerous.
Severe complications in infants include pneumonia, seizures, brain swelling (encephalopathy), dangerously high white blood cell counts, pulmonary hypertension, and heart failure. About 1 to 2 percent of hospitalized infant cases are fatal. Infants six months and younger account for over 80% of pertussis hospitalizations, and the youngest may require intensive care.
How It’s Diagnosed
Pertussis is difficult to catch early because its first stage mimics a cold. The most reliable diagnostic tool is a PCR test, which detects bacterial DNA from a swab taken from the back of the nose (nasopharynx). PCR works best during the first three weeks of cough, while enough bacterial DNA is still present. After the fourth week, the amount of DNA drops quickly, leading to a high risk of false negatives.
If you’ve already started antibiotics, PCR testing after five days of treatment is generally not useful because the medication clears enough bacteria to render the test unreliable. Bacterial culture is another option and remains the gold standard for confirmation, but it requires live bacteria and is less sensitive than PCR, so it’s most useful in the earliest stages of illness.
Treatment and Timing
Antibiotics are the standard treatment, with macrolide antibiotics (such as azithromycin, clarithromycin, or erythromycin) as the first-line choice. An alternative is available for people two months or older when macrolides can’t be used.
Timing matters enormously. If antibiotics are started during the first one to two weeks, before the severe coughing paroxysms begin, they can lessen the severity of symptoms. Started later, antibiotics still reduce how long you’re contagious to others but won’t change the course of the illness itself. The general treatment windows are within three weeks of cough onset for people one year and older, and within six weeks for infants under one year and pregnant women near their due date.
For very young infants under one month old, macrolide antibiotics are used with caution because of an association with a stomach condition called infantile hypertrophic pyloric stenosis. Even so, azithromycin remains the preferred choice in this age group because the risks of untreated pertussis are far greater.
Protecting Household Contacts
When someone in a household is diagnosed with pertussis, preventive antibiotics are recommended for all household contacts within 21 days of the infected person’s cough onset. This is especially critical for high-risk individuals: infants under 12 months (particularly those under 4 months), people with weakened immune systems, and those with moderate to severe asthma. The preventive antibiotics used are the same ones used for treatment.
Vaccination Schedule and Waning Immunity
Vaccination is the primary defense against pertussis. In the United States, the 2025 immunization schedule calls for a five-dose series of the DTaP vaccine for children: doses at 2, 4, and 6 months, a fourth dose at 15 to 18 months, and a fifth dose at 4 to 6 years. At age 11 to 12, children receive a single booster of the Tdap vaccine, which contains a lower dose of the pertussis component.
Pregnant women are recommended to receive one dose of Tdap during each pregnancy, preferably between 27 and 36 weeks of gestation. This allows the mother to pass protective antibodies to the baby before birth, covering the vulnerable gap before the infant’s own vaccination series begins at two months.
One important caveat: protection from the acellular pertussis vaccine wanes substantially within about five years after the last dose. A study published in the New England Journal of Medicine found that protection after the fifth DTaP dose dropped significantly over this period. This waning immunity is a major reason pertussis continues to circulate even in countries with high vaccination rates.
Why Pertussis Keeps Coming Back
Pertussis outbreaks follow a cyclical pattern, with peaks occurring roughly every three to five years regardless of vaccination type or coverage. This pattern holds across countries and continents. In Europe, the last pre-pandemic peaks were in 2016 and 2019, and another surge followed the pandemic period.
The global average pertussis incidence in 2023 was 23.6 cases per million people, but the numbers varied wildly by region. Europe saw a dramatic spike from 4.7 cases per million in 2022 to 104.4 in 2023. Some individual countries were hit especially hard: Croatia reported over 1,200 cases per million, and Denmark topped 1,000 per million.
Several factors fuel these cycles. Waning vaccine immunity means even vaccinated populations gradually lose protection. Natural immunity from infection also fades over time. And the years of reduced social mixing during the COVID-19 pandemic likely decreased exposure to the bacteria, leaving a larger pool of susceptible people once normal contact patterns resumed. The result is a disease that, despite decades of vaccination, remains one of the most common vaccine-preventable infections worldwide.