Whooping cough, also known as pertussis or the 100-day cough, is a highly contagious bacterial disease. Initial symptoms are usually similar to those of the common cold with a runny nose, fever, and mild cough, but these are followed by two or three months of severe coughing fits. Following a fit of coughing, a high-pitched whoop sound or gasp may occur as the person breathes in. The violent coughing may last for 10 or more weeks, hence the phrase "100-day cough". A person may cough so hard that they vomit, break ribs, or become very tired from the effort. Children less than one year old may have little or no cough and instead have periods where they cannot breathe. The time between infection and the onset of symptoms is usually seven to ten days. Disease may occur in those who have been vaccinated, but symptoms are typically milder.

Pertussis is caused by the bacterium Bordetella pertussis. It is spread easily through the coughs and sneezes of an infected person. People are infectious from the start of symptoms until about three weeks into the coughing fits. Those treated with antibiotics are no longer infectious after five days. Diagnosis is by collecting a sample from the back of the nose and throat. This sample can then be tested by either culture or by polymerase chain reaction.

Prevention is mainly by vaccination with the pertussis vaccine. Initial immunization is recommended between six and eight weeks of age, with four doses to be given in the first two years of life. Protection from pertussis decreases over time, so additional doses of vaccine are often recommended for older children and adults. Vaccination during pregnancy is highly effective at protecting the infant from pertussis during their vulnerable early months of life, and is recommended in many countries. Antibiotics may be used to prevent the disease in those who have been exposed and are at risk of severe disease. In those with the disease, antibiotics are useful if started within three weeks of the initial symptoms, but otherwise have little effect in most people. In pregnant women and children less than one year old, antibiotics are recommended within six weeks of symptom onset. Antibiotics used include erythromycin, azithromycin, clarithromycin, or trimethoprim/sulfamethoxazole. Evidence to support interventions for the cough, other than antibiotics, is poor. About 50% of infected children less than a year old require hospitalization and nearly 0.5% (1 in 200) die.

An estimated 16.3 million people worldwide were infected in 2015. Most cases occur in the developing world, and people of all ages may be affected. In 2015, pertussis resulted in 58,700 deaths – down from 138,000 deaths in 1990. Outbreaks of the disease were first described in the 16th century. The bacterium that causes the infection was discovered in 1906. The pertussis vaccine became available in the 1940s.