Influenza (abbreviated as "flu") is an acute respiratory infectious disease caused by influenza virus. It is mainly transmitted through droplets of respiratory secretions, and can also be transmitted through direct or indirect contact with mucous membranes such as the mouth, nose, and eyes. Influenza viruses can be divided into four types: A, B, C, and D (or A, B, C, and D). Currently, the main types of influenza A viruses that infect humans are the H1N1 and H3N2 subtypes and the Victoria and Yamagata strains of influenza B viruses, which can cause seasonal epidemics every year.
The main symptoms of influenza are sudden high fever (39℃~40℃), headache, muscle aches and general discomfort, sore throat, dry cough, nasal congestion, runny nose, discomfort behind the sternum, facial flushing, conjunctival congestion, etc. Among the complications, pneumonia is the most common, and other complications include nervous system damage, heart damage, myositis and rhabdomyolysis, shock, etc. Children with influenza are more likely to have laryngitis, otitis media, and bronchitis than ordinary people. Influenza is prone to outbreaks in places where people gather, such as schools, childcare institutions, and nursing homes. People are generally susceptible to influenza viruses, and high-risk groups such as pregnant women, infants, the elderly, and patients with chronic diseases are more seriously harmed by influenza.