流行性感冒 (Influenza)，通常简称为流感，为一种由流感病毒造成的传染性疾病。症状可轻可重，最常见者为高烧、流鼻水、喉咙痛、肌肉酸痛、头痛、咳嗽、以及疲倦感。通常在接触病毒2天后发病，症状最多持续一周。然而咳嗽可能持续超过两周。在孩童可能有恶心、呕吐，但这在成人并不常见。恶心及呕吐常为非相关之感染性肠胃炎，有时被称为肠胃型感冒。流感并发症可能有病毒性肺炎、次发性细菌性肺炎、鼻窦感染、以及造成其他疾病恶化（如气喘或心脏衰竭）。

可感染人类的流感病毒共有三种。通常病毒借由咳嗽或喷嚏的产生的飞沫传播，且发生在近距离接触时。此外，病毒也可借由接触到受染污的物体表面，再碰触口或眼睛后传播。受感染后在发病前后，均可能具有传染性。诊断可由喉咙、痰液、或鼻腔等的病毒测试作确认。目前已有数种快速筛检方法，然而快速筛检方法仍有伪阴性（即使受感染，检测结果仍显示为未感染的阴性）的可能。而使用聚合酶链式反应检测病毒RNA，为较准确的检验方法。

Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus. Symptoms can be mild to severe. The most common symptoms include: a high fever, runny nose, sore throat, muscle pains, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be nausea and vomiting, but these are not common in adults. Nausea and vomiting occur more commonly in the unrelated infection gastroenteritis, which is sometimes inaccurately referred to as "stomach flu" or "24-hour flu". Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.