Bordetella pertussis is the bacterium that causes whooping cough, one of the most contagious respiratory infections in humans. It’s a tiny, rod-shaped, Gram-negative bacterium that lives exclusively in the human respiratory tract, meaning people are its only natural host. With a basic reproduction number around 10.6 (meaning one infected person typically spreads it to about 10 or 11 others in an unvaccinated population), it ranks among the most transmissible bacterial infections known.
How It Infects the Airways
B. pertussis measures roughly 0.8 by 0.4 micrometers, far too small to see without a microscope. It doesn’t form spores or survive long in the environment. Instead, it spreads through respiratory droplets when an infected person coughs, sneezes, or shares breathing space with others. Once it lands on the mucous membranes lining the nose and throat, it attaches to the cells there and begins producing toxins that damage the airway lining and suppress the local immune response.
An infected person can spread the bacterium for roughly 15 days during the infectious period. The most contagious window overlaps with the earliest stage of illness, when symptoms still look like a common cold and most people have no idea they’re carrying pertussis.
The Three Stages of Whooping Cough
Pertussis follows a distinctive pattern that unfolds over weeks to months, divided into three clinical stages.
Stage 1: Catarrhal
The illness starts with a runny nose, mild cough, and possibly a low-grade fever. It looks and feels almost identical to a regular cold, which is why it so often goes unrecognized. Over the course of one to two weeks, the cough gradually worsens. In infants, this stage may also include episodes of apnea, where the baby briefly stops breathing.
Stage 2: Paroxysmal
This is the stage that gives whooping cough its name. Coughing attacks come in rapid, uncontrollable bursts (paroxysms) followed by a desperate inhale that produces a high-pitched “whoop.” These fits can be violent enough to cause vomiting, exhaustion, and even a bluish tint to the skin from lack of oxygen. They tend to be worse at night, averaging about 15 episodes per 24 hours. The attacks increase in frequency during the first one to two weeks, plateau for another two to three weeks, then slowly taper off.
Stage 3: Convalescent
Recovery is gradual. The paroxysmal coughing becomes less frequent and usually disappears within two to three weeks. But the airways remain irritable for a long time afterward. Many people find that catching an unrelated cold or respiratory virus in the following months can trigger the paroxysmal cough all over again, which is why pertussis earned the old nickname “the hundred-day cough.”
How It’s Diagnosed
Diagnosing pertussis depends heavily on timing. The most reliable method is a PCR test performed on a swab taken from deep inside the nose (a nasopharyngeal swab). PCR works best during the first three weeks of cough, when bacterial DNA is still abundant. After the fourth week, bacterial DNA levels drop rapidly and the test becomes much less reliable, often returning falsely negative results. Traditional bacterial culture is another option, though PCR offers faster turnaround and better sensitivity.
Because the earliest stage mimics a common cold, many cases aren’t suspected until the distinctive coughing fits begin, which already puts diagnosis into a narrow testing window.
Treatment and What to Expect
Antibiotics can make a real difference, but timing matters enormously. If treatment begins during the first one to two weeks of illness, before the severe coughing paroxysms set in, it can lessen symptom severity and shorten the contagious period. Started later, antibiotics still help reduce transmission to close contacts but won’t change the course of the illness itself. The coughing fits will run their course regardless.
The primary antibiotics used belong to a class called macrolides. For very young infants under one month old, treatment requires careful monitoring because of a small risk of a stomach condition, but the danger of untreated pertussis in newborns is severe enough that treatment is still recommended. For anyone two months or older who can’t tolerate macrolides, an alternative antibiotic option exists.
Close household contacts are typically offered preventive antibiotics as well, regardless of their vaccination status, to stop the chain of transmission.
Vaccination and Waning Immunity
Vaccination remains the most effective protection against pertussis. Children who receive all five scheduled doses of the childhood vaccine (DTaP) are 98% protected within the year following their last dose. But that protection fades. Five years after the final childhood dose, effectiveness drops to about 71%.
This waning immunity is a central challenge. The acellular vaccines used today in the United States don’t provide protection that lasts as long as the older whole-cell vaccines they replaced. That’s one reason booster doses (Tdap) are recommended for adolescents, pregnant people during each pregnancy, and adults who haven’t previously received one. Vaccinating during pregnancy is particularly important because it allows protective antibodies to pass to the newborn, covering the vulnerable gap before the baby is old enough to start their own vaccine series.
Why Cases Are Rising Again
Pertussis never disappeared. Before vaccines became widely available in the 1940s, it was one of the leading causes of childhood illness. Vaccination drove case numbers down dramatically, but reported cases have been trending upward since the 1980s. The COVID-19 pandemic temporarily suppressed pertussis transmission, as it did for many respiratory infections, but the reprieve was short-lived.
In 2024, the United States saw a sharp resurgence. Preliminary data from the CDC show more than six times as many reported cases compared to 2023, exceeding even the pre-pandemic levels seen in 2019. Cases peaked around November 2024 and have been trending down since, though numbers in 2025 remain elevated compared to the years immediately before the pandemic.
Several factors are driving this pattern. Waning vaccine immunity means even vaccinated individuals can become susceptible a few years after their last dose. Improved diagnostic testing and better reporting capture cases that might have been missed in earlier decades. And the bacterium itself continues to circulate widely, finding gaps wherever immunity has faded. The CDC expects pertussis to affect both vaccinated and unvaccinated populations as transmission patterns normalize, reinforcing the importance of staying current on booster doses.