百日咳（Pertussis）是一种高度传染性的细菌性传染病。一开始的病征通常类似于普通感冒，会有流鼻水、发烧和轻微咳嗽等症状。之后几周会伴随着剧烈咳嗽，就连呼吸时也会发出高音调的「呼哮」声（whoop）。这种现象可能持续超过一百天或是十周。被感染者有可能因为咳得太严重而有想吐、感到疲倦的情况，甚者咳到肋骨断裂 。未满足岁的幼儿咳嗽的情况可能并不明显，但却会出现好几次呼吸暂时停止的情形。一般而言，从感染百日咳到病发的潜伏期为7到10天，而百日咳也会在那些已经打过疫苗但症状比较轻微的人身上发现。

百日咳是由百日咳杆菌引起的疾病，可借由已感染者的咳嗽或喷嚏而传播给他人，是空气传播疾病的一种 。病患对他人的传染性是从疾病开始一直延续到约三周后开始产生剧烈咳嗽后，但若以抗生素治疗五天后将不再具备传染性 。针对此疾病的检验主要是使用鼻咽腔卷棉拭子来进行，检体可再经由细菌培养或是聚合酶连锁反应来检验并确认。

Pertussis (also known as whooping cough or 100-day cough) is a highly contagious bacterial disease. Initially, symptoms are usually similar to those of the common cold with a runny nose, fever, and mild cough. This is then followed by weeks of severe coughing fits. Following a fit of coughing, a high-pitched whoop sound or gasp may occur as the person breathes in. The 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 do not 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.