Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin from one to four days after exposure to the virus and last for about 2–8 days. Diarrhea and vomiting can occur, particularly in children. Influenza may progress to pneumonia, which can be caused by the virus or by a subsequent bacterial infection. Other complications of infection include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease.

इन्फ़्लुएन्ज़ा एक विशेष समूह के वायरस के कारण मानव समुदाय में होनेवाला एक संक्रामक रोग है। इसमें ज्वर और अति दुर्बलता विशेष लक्षण हैं। फुफ्फुसों के उपद्रव की इसमें बहुत संभावना रहती है। यह रोग प्राय: महामारी के रूप में फैलता है। बीच-बीच में जहाँ-तहाँ रोग होता रहता है।