Influenza and its symptoms

Influenza commonly known as "flu" is an infectious disease caused by influenza virus.Symptoms range from mild to severe and often include fever,runny nose, sore throat, muscle pain,headache, cough and fatigue.These symptoms start one to four days (usually two) after exposure to the virus and last about 2-8 days.Diarrhea and vomiting may occur, especially in children. Influenza can progress to pneumonia, which can be caused by a virus or subsequent bacterial infection.Other complications of infection include acute respiratory distress syndrome, meningitis, encephalitis, and exacerbation of pre-existing health problems such as asthma and cardiovascular disease.There are four types of influenza viruses, called influenza A, B, C, and D viruses.Waterfowl are a major source of influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs.Influenza B virus (IBV) and influenza C virus (ICV) mainly infect humans, and influenza D virus (IDV) mainly infects cattle and pigs.IAV and IBV circulate in humans and cause seasonal epidemics, while ICV causes mild infections, mainly in children.IDV can infect humans but does not cause disease.In humans, influenza viruses are mainly spread through respiratory droplets produced by coughing and sneezing.Can also be transmitted by aerosols and virus-contaminated intermediate objects and surfaces.

Washing your hands often and covering your coughs and sneezes can reduce transmission.Getting vaccinated every year can help prevent the flu.Influenza viruses, especially IAV, evolve rapidly, so flu vaccines are regularly updated to match circulating flu strains. Vaccines currently in use provide protection against the H1N1 and H3N2 subtypes of IAV and one or both IBV subtypes. Influenza infection is diagnosed by laboratory methods such as antibody or antigen tests and polymerase chain reaction (PCR) to determine viral nucleic acid.The disease can be treated with supportive measures and, in severe cases, antiviral drugs such as oseltamivir.In healthy individuals, influenza is usually self-limited and rarely fatal, but can be fatal in high-risk groups.In a typical year, 5-15% of the population gets the flu.There are 3-5 million severe cases and up to 650,000 respiratory-related deaths worldwide each year.Deaths most commonly occur in high-risk groups, including young children, the elderly and people with chronic diseases.In temperate regions of the world, influenza case numbers peak in winter, while in tropical regions influenza can occur year-round.Every 10 to 50 years since the late 1800s, large outbreaks of new strains of influenza that spread around the globe, known as pandemics, have occurred.There have been five flu pandemics since 1900:the Spanish flu of 1918-1920, which was the worst influenza pandemic ever, the Asian flu of 1957, the Hong Kong flu of 1968, the Russian flu of 1977 and the swine flu of 2009 influenza pandemic.

Signs and symptoms

The time between exposure to the virus and the onset of symptoms is called the incubation period and is 1-4 days,most commonly 1-2 days.However, many infections are asymptomatic.Symptom onset is sudden and initial symptoms are largely nonspecific and include fever, chills, headache, muscle pain or aches, malaise, loss of appetite, lack of energy/fatigue, and confusion.These symptoms are often accompanied by respiratory symptoms such as dry cough, sore or dry throat, hoarseness, and nasal congestion or runny nose. Cough is the most common symptom.Gastrointestinal symptoms, including nausea, vomiting, diarrhea and gastroenteritis,may also occur, especially in children. Standard flu symptoms usually last 2-8 days.A 2021 study showed that influenza can cause long-lasting symptoms in a manner similar to long-term COVID.Symptomatic infection is usually mild and limited to the upper respiratory tract, but progression to pneumonia is relatively common.Pneumonia may be caused by a primary viral infection or a secondary bacterial infection.Primary pneumonia is characterized by rapidly progressive fever, cough, difficulty breathing, and low oxygen levels that cause the skin to turn blue.This is especially common in people with underlying cardiovascular disease such as rheumatic heart disease.Secondary pneumonia usually has a 1-3 week period of symptom improvement  followed by recurrent fever, sputum production, and pulmonary fluid accumulation but may also occur several days after the onset of influenza symptoms.Approximately one third of primary pneumonia cases develop secondary pneumonia, most commonly caused by Streptococcus pneumoniae and Staphylococcus aureus.