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By Mayo Clinic staffWhooping cough is a respiratory infection affecting mostly the windpipe (trachea) and the tubes branching off from the windpipe (bronchi). It's caused by the Bordetella pertussis bacterium, which is transmitted through droplets that are coughed or sneezed into the air by someone who's already infected. Whooping cough is most contagious early on, but the possibility of spreading the illness remains until the infection clears completely.
Once inside your airways, the bacteria multiply and produce toxins that interfere with your respiratory tract's ability to sweep away germs. Thick mucus accumulates inside your airways, causing uncontrollable coughing.
The bacteria also cause inflammation that narrows breathing tubes in your lungs. This narrowing leaves you gasping for air — sucking in air with a high-pitched "whoop" — after a fit of coughing.