流行性感冒( 簡稱流感) 是由流感病毒引致的急性呼吸道疾病，好發於冬季，而在熱帶地區則可能全年發生。在台灣，一般典型季節性流感的流行季節於每年11 月開始至隔年3 月，於隔年1 月至2 月達到流行高峰。流感病毒可分為A、B、C、D 四種型別。其中A 型流感病毒又可以依據病毒表面血球凝集素(hemagglutinin，H 抗原) 及神經胺酸酶(neuraminidase，N 抗原) 兩種抗原再細分為不同亞型，此抗原變化與流感的流行有關。人類目前流行的季節性流感病毒主要為A 型H1N1、A 型H3N2 以及B 型流感。流感病毒在自然演化過程中經常發生突變產生新的病毒株，稱為抗原微變(antigenic drift)，世界衛生組織會根據前一次流感季節病毒株預測應採用的流感疫苗組合。當流感病毒出現H 抗原或N 抗原的型別轉變時，稱為抗原移型(antigenic shift)，而導致可以造成大流行的新型流感病毒。流感的潛伏期約1 至4 天，症狀包括發燒、頭痛、流鼻水、喉嚨痛、咳嗽、肌肉酸痛及疲倦無力等。高危險群的病患，如老年人、懷孕者、糖尿病、慢性心肺疾病、肝硬化及免疫不全病人可能引發嚴重併發症，甚至導致死亡。最常見的流感併發症為肺炎，其他還包括中耳炎、鼻竇炎、腦炎、心肌炎、心包膜炎等。由於流感對於個人健康、社會及經濟都造成極大影響，除了戴口罩及勤洗手，每年接種流感疫苗是預防季節性流感和其併發症最有效方式。

Influenza is an acute respiratory disease caused by the influenza virus, which occurs in winter and may occur throughout the year in tropical regions. In Taiwan, the typical epidemic season of seasonal influenza starts from November to March of the following year, and generally reaches the peak of epidemic from January to February of the following year. Influenza viruses can be divided into four types: A, B, C, and D. Influenza A virus can be further subdivided into different subtypes according to hemagglutinin (H antigen) and neuraminidase (N antigen) on the surface of the virus. The change of this antigen is related to influenza epidemic. The seasonal influenza viruses currently circulating in humans are mainly type A H1N1, type A H3N2, and type B influenza. Because seasonal influenza virus antigenic drift (antigenic drift) can generate new strains, the World Health Organization will recommend an annual influenza vaccine combination for the flu season. When an influenza virus undergoes an antigenic shift, which can effectively pass it from person to person, a new virus that can cause an influenza pandemic will be produced. Influenza incubation period is about 1 to 4 days. Symptoms include fever, headache, runny nose, sore throat, cough, muscle aches and fatigue. Patients with high-risk groups, such as the elderly, pregnant women, chronic heart and lung diseases, and immunocompromised patients may cause serious complications and even death; the most common complication is pneumonia, and others may be complicated by otitis media, sinusitis, and encephalitis, myocarditis, pericarditis. An influenza pandemic has a great impact on personal health, society and the economy. In addition to wearing a mask and hand hygiene, annual influenza vaccination can safely and effectively prevent seasonal influenza and its complications.