<unk> , commonly known as "the<unk> ", is an infectious disease caused by<unk>  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 (typically two days) and last for about 2–8 days. Diarrhea and vomiting can occur, particularly in children.<unk>  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. There are four types of<unk>  virus, termed<unk>  viruses A, B, C, and D. Aquatic birds are the primary source of<unk>  A virus (IAV), which is also widespread in various mammals, including humans and pigs.<unk>  B virus (IBV) and<unk>  C virus (ICV) primarily infect humans, and<unk>  D virus (IDV) is found in cattle and pigs. IAV and IBV circulate in humans and cause seasonal epidemics, and ICV causes a mild infection, primarily in children. IDV can infect humans but is not known to cause illness. In humans,<unk>  viruses are primarily transmitted through respiratory droplets produced from coughing and sneezing. Transmission through aerosols and intermediate objects and surfaces contaminated by the virus also occur. Frequent hand washing and covering one's mouth and nose when coughing and sneezing reduce transmission. Annual vaccination can help to provide protection against<unk> .<unk>  viruses, particularly IAV, evolve quickly, so<unk>  vaccines are updated regularly to match which<unk>  strains are in circulation. Vaccines currently in use provide protection against IAV subtypes H1N1 and H3N2 and one or two IBV subtypes.<unk>  infection is diagnosed with laboratory methods such as antibody or antigen tests and a polymerase chain reaction (PCR) to identify viral nucleic acid. The disease can be treated with supportive measures and, in severe cases, with antiviral drugs such as oseltamivir. In healthy individuals,<unk>  is typically self-limiting and rarely fatal, but it can be deadly in high-risk groups. In a typical year, 5–15% of the population contracts<unk> . There are 3–5 million severe cases annually, with up to 650,000 respiratory-related deaths globally each year. Deaths most commonly occur in high-risk groups, including young children, the elderly, and people with chronic health conditions. In temperate regions of the world, the number of<unk>  cases peaks during winter, whereas in the tropics<unk>  can occur year-round. Since the late 1800s, large outbreaks of novel<unk>  strains that spread globally, called pandemics, have occurred every 10–50 years. Five<unk>  pandemics have occurred since 1900: the Spanish<unk>  in 1918–1920, which was the most severe<unk>  pandemic, the Asian<unk>  in 1957, the Hong Kong<unk>  in 1968, the Russian<unk>  in 1977, and the swine<unk>  pandemic in 2009.
Influenza
Q fever
Swine influenza
Spanish flu
Bird Flu
London flu
Freshers' flu
Polymer fume fever