In temperate regions of the world, the number of influenza cases peaks during winter, whereas in the tropics influenza can occur year-round. Deaths most commonly occur in high-risk groups, including young children, the elderly, and people with chronic health conditions. There are 3–5 million severe cases annually, with up to 650,000 respiratory-related deaths globally each year. In a typical year, 5–15% of the population contracts influenza. In healthy individuals, influenza is typically self-limiting and rarely fatal, but it can be deadly in high-risk groups. The disease can be treated with supportive measures and, in severe cases, with antiviral drugs such as oseltamivir. Influenza infection is diagnosed with laboratory methods such as antibody or antigen tests and a polymerase chain reaction ( PCR) to identify viral nucleic acid. Vaccines provide protection against IAV subtypes H1N1 and H3N2 and one or two IBV subtypes. Influenza viruses, particularly IAV, evolve quickly, so flu vaccines are updated regularly to match which influenza strains are in circulation. Annual vaccination can help to provide protection against influenza. Transmission through aerosols and intermediate objects and surfaces contaminated by the virus also occur.įrequent hand washing and covering one's mouth and nose when coughing and sneezing reduce transmission. In humans, influenza viruses are primarily transmitted through respiratory droplets produced from coughing and sneezing. IDV can infect humans but is not known to cause illness. IAV and IBV circulate in humans and cause seasonal epidemics, and ICV causes a mild infection, primarily in children. Influenza B virus (IBV) and Influenza C virus (ICV) primarily infect humans, and Influenza D virus (IDV) is found in cattle and pigs. Aquatic birds are the primary source of Influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs. There are four types of influenza virus: A, B, C, and D. Other complications of infection include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease. Influenza may progress to pneumonia, which can be caused by the virus or by a subsequent bacterial infection. Diarrhea and vomiting can occur, particularly in children.

These symptoms begin from one to four days after exposure to the virus (typically two days) and last for about 2–8 days. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. Influenza, commonly known as ' the flu', is an infectious disease caused by influenza viruses.

Influenza virus, magnified approximately 100,000 timesįever, runny nose, sore throat, muscle pain, headache, coughing, fatigueģ–5 million severe cases per year

Define antigenic shift