[51][52], The first convincing record of an influenza pandemic was chronicled in 1510; it began in East Asia before spreading to North Africa and then Europe. Residents of nursing homes and other long-term care facilities, People who are pregnant or plan to be pregnant during flu season, People who have chronic illnesses, such as asthma, heart disease, kidney disease, liver disease and diabetes, People with a body mass index (BMI) of 40 or higher, Vomiting and diarrhea, but this is more common in children than adults, Difficulty breathing or shortness of breath, Pale, gray or blue-colored skin, lips or nail beds depending on skin color, Children between 2 and 17 years old who are taking aspirin or a salicylate-containing medication, Kids 2 to 4 years old who have had asthma or wheezing in the past 12 months. [24] Transmission to poultry typically occurs in backyard farming and live animal markets where multiple species interact with each other. [23] It was caused by a novel H1N1 strain that was a reassortment of human, swine, and avian influenza viruses. [45] This H1N1 strain was antigenically similar to the H1N1 strains that circulated prior to 1957. Hibberd PL. [1][39], Nucleic acid-based tests (NATs) amplify and detect viral nucleic acid. 21th ed. These trusted information partners have more on this topic. Common side effects [23][54] Whether the 1977 "pandemic" deserves to be included in the natural history of flu pandemics is debatable. The categories refer to molecular characteristics of a virus and the virus ability to cause disease and mortality in chickens in a laboratory setting pdf icon[2.55 MB, 64 Pages]external icon. [1][11][20] To counter the immune response, influenza viruses encode various non-structural proteins, including NS1, NEP, PB1-F2, and PA-X, that are involved in curtailing the host immune response by suppressing interferon production and host gene expression. Secondary pneumonia typically has a period of improvement in symptoms for 13 weeks[13] followed by recurrent fever, sputum production, and fluid buildup in the lungs,[1] but can also occur just a few days after influenza symptoms appear. Or you can pick up the germs from an object such as a telephone or computer keyboard and then transfer them to your eyes, nose or mouth. Symptoms of Influenza A include fever and chills, sneezing, coughing and a sore throat. [19] Influenza viruses also encode various accessory proteins, such as PB1-F2 and PA-X, that are expressed through alternative open reading frames[1][20] and which are important in host defense suppression, virulence, and pathogenicity. [50] In 1648, it was discovered that horses also experience influenza. [17] Likewise, ICV primarily infects humans but has been observed in pigs, dogs, cattle, and dromedary camels. [37], In health care settings, infected individuals may be cohorted or assigned to individual rooms. Nearly all IAVs are apparently descended from ancestral viruses in birds. Influenza virus C causes mild sickness, and influenza virus D causes . H1N1, which caused Spanish Flu in 1918 and Swine Flu in 20091 1. In 2013, Taiwan reported the first known human infection with LPAI A(H6N1) virus. [45] During the 1918 pandemic, the respiratory route of transmission was clearly identified[23] and influenza was shown to be caused by a "filter passer", not a bacterium, but there remained a lack of agreement about influenza's cause for another decade and research on influenza declined. Cases disproportionately occur in children, but most severe causes are among the elderly, the very young,[1] and the immunocompromised. There are many subtypes of avian influenza viruses, but only some strains of five subtypes have been known to infect humans: H5N1, H7N3, H7N7, H7N9, and H9N2. 1998-2022 Mayo Foundation for Medical Education and Research (MFMER). Binding experiments with radioactively labelled influenza C virions were carried out to investigate the interaction of the virus with human erythrocytes. asthma), kidney, liver, blood, neurological, or metabolic (e.g. Once bound to these proteins, antibodies block virions from binding to cellular receptors, neutralizing the virus. Most animal viruses are classified as icosahedral and are nearly spherical in shape. Influenza virus RNA is what is known as 'negative sense RNA'. The standard influenza symptoms typically last for 28 days. As well as infecting people, the influenza A virus can infect animals, including birds and pigs. Sporadic avian influenza A(H9N2) virus infections have been reported in people with mild upper respiratory tract illness, although some infections have resulted in death. Influenza viruses circulate year-round, but the number of new influenza cases generally increases in the cooler months and tapers off in the warmer months of the year. This study represents the first healthy volunteer influenza challenge model using a GMP-produced wild-type virus under an IND. Illness during infection is primarily the result of lung inflammation and compromise caused by epithelial cell infection and death, combined with inflammation caused by the immune system's response to infection. Fatigue and malaise may last for several weeks after recovery, and healthy adults may experience pulmonary abnormalities that can take several weeks to resolve. Influenza type A viruses are classified into subtypes according to the combinations of different virus surface proteins hemagglutinin (HA) and neuraminidase (NA). Table 1: Influenza Virus Testing Methods; Method 1 Types Detected Acceptable Specimens 2 Test Time CLIA Waived 3; Rapid Influenza Diagnostic Tests 4 (antigen detection): A and B: NP 5 swab, aspirate or wash, nasal swab, aspirate or wash, throat swab <15 min. AskMayoExpert. There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as the flu season) almost every winter in the United States. H3N2 has a much broader host range and can reassort with H1N1 and H5N1. Pharmaceutical measures are effective but may not be available in the early stages of an outbreak. [39] Rapid influenza diagnostic tests (RIDTs) are a simple way of obtaining assay results, are low cost, and produce results quickly, at less than 30 minutes, so they are commonly used, but they can't distinguish between IAV and IBV or between IAV subtypes and are not as sensitive as nucleic-acid based tests. [53][54] Shope's research reinvigorated research on human influenza, and many advances in virology, serology, immunology, experimental animal models, vaccinology, and immunotherapy have since arisen from influenza research. [20], Two key processes that influenza viruses evolve through are antigenic drift and antigenic shift. [1] The first antiviral drug against influenza, amantadine, was approved for use in 1966, with additional antiviral drugs being used since the 1990s. There are 35 million severe cases annually, with up to 650,000 respiratory-related deaths globally each year. Influenza D is the only type of influenza virus that mainly affects cattle with frequent spillover to other species. There are 18 different known H antigens (H1 to H18) and 11 different known N antigens (N1 to N11). Antigenic drift is when an influenza virus's antigens change due to the gradual accumulation of mutations in the antigen's (HA or NA) gene. These antigens also determine their infectious properties . This year, the vaccine will be available as an injection and as a nasal spray. The first human infections with A(H10N8) virus were reported in China in December 2013. [2] Flu seasons are different in the northern and southern hemisphere, so the WHO meets twice a year, one for each hemisphere, to discuss which strains should be included in flu vaccines based on observation from HA inhibition assays. Egypt reported the first human infections with A(H10N7) virus in 2004, while Australia reported human infections with A(H10N7) virus in March 2010. There are nine known subtypes of A(H5) viruses pdf icon[1.34 KB, 15 Pages]external icon[A(H5N1), A(H5N2), A(H5N3), A(H5N4), A(H5N5), A(H5N6), A(H5N7), A(H5N8), and A(H5N9)]. A third wave with lower mortality occurred in many places a few months after the second. Vaccines currently in use provide protection against IAV subtypes H1N1 and H3N2 and one or two IBV subtypes. In: Goldman-Cecil Medicine. the influenza virus is divided into four types (a, b, c and d) by antigenic differences in their core proteins, including matrix protein (m1) and nucleoprotein (np). You can review and change the way we collect information below. Influenza A viruses are further categorized into subtypes based on the 2 major surface protein antigens . Most of these tests take a few hours,[39] but rapid molecular assays are as fast as RIDTs. These two subtypes are included in the seasonal flu vaccine each year. While most healthy people recover from the flu in about a week, children, the elderly, and people with weakened immune systems or . Viral non-structural proteins including NS1, PB1-F2, and PA-X regulate host cellular processes to disable antiviral responses. Influenza (the flu) is a viral infection that causes symptoms like fever and chills, sneezing, coughing and a sore throat. Mayo Clinic; 2020. For mammalian influenza viruses and low pathogenic AIVs, cleavage is extracellular, which limits infection to cells that have the appropriate proteases, whereas for highly pathogenic AIVs, cleavage is intracellular and performed by ubiquitous proteases, which allows for infection of a greater variety of cells, thereby contributing to more severe disease. [1] Equine H3N8 perferentially binds to -2,3 sialic acids, so horses are usually considered dead-end hosts, but transmission to dogs and camels has occurred, raising concerns that horses may be mixing vessels for reassortment. Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. This is why yearly vaccination is recommended. https://www.uptodate.com/contents/search. [10][17] IDV causes an influenza-like illness in pigs but its impact in its natural reservoir, cattle, is relatively unknown. Consequently, H1N1, H3N2, and both IBV lineages have been in circulation in seasonal form since the 2009 pandemic. Large outbreaks of ICV infection can occur, so incidence varies significantly. Among humans, annual epidemic influenza illness is caused by two types of influenza viruses, influenza A and influenza B . There are 18 different H subtypes in influenza A that run from influenza H1 to H18. Influenza infection is diagnosed with laboratory methods such as antibody or antigen tests and a polymerase chain reaction (PCR) to identify viral nucleic acid. Accessed Sept. 16, 2022. Influenza A viruses (IAVs) and type B viruses (IBVs) contain 8, negative-sense, single-stranded viral RNA (vRNA) gene segments (Figure 1 A) ( 3, 4 ), which encode transcripts for 10 essential viral proteins, as well as several strain-dependent accessory proteins (Figure 1 B). IAV has evolved various strategies to counteract the innate immune response, using different viral proteins. IDV can infect humans but is not known to cause illness. [1] In pigs and horses, management of influenza is dependent on vaccination with biosecurity. There are nine known subtypes of A(H7) viruses pdf icon[1.34 KB, 15 Pages]external iconA(H7N1), A(H7N2), A(H7N3), A(H7N4), A(H7N5), A(H7N6), A(H7N7), A(H7N8), and A(H7N9)]. [1] There is, however, an "original antigenic sin", in which the first HA subtype a person is exposed to influences the antibody-based immune response to future infections and vaccines. During an outbreak of influenza in 1743 that started in Italy and spread throughout Europe, the word reached the English language and was anglicized in pronunciation. Dr. Poland: Every American, age six months and older, get a flu vaccine. There are four types of influenza (flu) viruses: A, B, C and D. Wild aquatic birds, including gulls, terns, and shorebirds, and wild waterfowl, such as ducks, geese and swans are considered reservoirs (hosts) for avian influenza A viruses. health information, we will treat all of that information as protected health These proteins are called hemagglutinin (HA) and neuraminidase (NA). Although bird flu can be severe, the number of cases found in humans has been small. [47] By the time of the 1889 pandemic, which may have been caused by an H2N2 strain,[55] the flu had become an easily recognizable disease. How to protect yourself & others. When discharging patients to care homes, it is important to take care if there is a known influenza outbreak. healthdirect's information and advice are developed and managed within a rigorous Influenza. [36] Other methods to limit influenza transmission include staying home when sick,[1] avoiding contact with others until one day after symptoms end,[9] and disinfecting surfaces likely to be contaminated by the virus, such as doorknobs. The A(H10) virus subtypes known to have caused human infections include A(H10N3), A(H10N7), and A(H10N8). Please check and try again, Please enter recipient's It is especially common among those who have an underlying cardiovascular disease such as rheumatic heart disease. The most frequently identified A(H7) viruses associated with human infections are avian influenza A(H7N9) viruses, which were first detected inChina in 2013. Overview Flu (influenza) is an infection of the nose, throat and lungs, which are part of the respiratory system. About 10 per cent of children (1 out of every 10) who have an influenza vaccine experience swelling, redness, pain at the injection site that lasts one or two days. H5N1 and H7N9 cause severe lower respiratory tract illness, whereas other AIVs such as H9N2 cause a more mild upper respiratory tract illness, commonly with conjunctivitis. These methods tend to be relatively inexpensive and fast but are less reliable than nucleic-acid based tests. Subtypes of Influenza A Viruses [8][3] Other manufacturing methods include an MDCK cell culture-based inactivated vaccine and a recombinant subunit vaccine manufactured from baculovirus overexpression in insect cells. [17][18] IDV is distantly related to ICV. In most cases you can treat the symptoms of a mild flu yourself by resting at home. We are a government-funded service, providing quality, approved health information and advice. There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as flu season) almost every winter in the United States. [40] Aspirin is not recommended to treat influenza in children due to an elevated risk of developing Reye syndrome. In the northern hemisphere, this is from October to May (more narrowly December to April[11]), and in the southern hemisphere, this is from May to October (more narrowly June to September[11]). [11], In temperate regions, the number of influenza cases varies from season to season. You can inhale the droplets directly. Human infections with other subtypes, such as A(H6N1), A(H10N3), A(H10N7), and A(H10N8), have been detected in small numbers of people. Accessed Sept. 16, 2022. Similarly, an A(H5N1) virus has an HA 5 protein and an NA 1 protein. [20] AIVs prefer sialic acids with an -2,3 linkage, which are most common in birds in gastrointestinal epithelial cells[1][15][3] and in humans in the lower respiratory tract. [15] Typically, infection with an AIV has an incubation period of 35 days but can be up to 9 days. [1] Complications affecting the cardiovascular system are rare and include pericarditis, fulminant myocarditis with a fast, slow, or irregular heartbeat, and exacerbation of pre-existing cardiovascular disease. All information these cookies collect is aggregated and therefore anonymous. information submitted for this request. [1][15][3] For N1 subtypes with the "G147R" mutation and N2 subtypes, the NA protein can initiate entry. abbreviations: acip = advisory committee on immunization practices; fda = food and drug administration; ha = hemagglutinin; iiv4 = inactivated influenza vaccine, quadrivalent; im = intramuscular; laiv4 = live attenuated influenza vaccine, quadrivalent; mdv = multidose vial; nas = intranasal; pfs = prefilled syringe; riv4 = recombinant influenza Dr. Poland: The symptoms of COVID and influenza overlap almost exactly in their initial manifestations with the exception of the loss of smell, loss of taste. And if a COVID-19 vaccine or booster and a flu vaccination are due at the same time, the CDC reports that you can get vaccinated for both in one visit. [2][66] mRNA vaccines to provide protection against influenza are also under research. This pandemic had a high attack rate, but the mortality rate was low. [1][45] Influenza may therefore occur year-round in these regions. https://www.cdc.gov/flu/highrisk/index.htm. Four types of influenza virus infect humansA, B, C and D. Among the four categories, the A-type virus has the ability to cause a pandemic. Avian Influenza (AI) type A viruses are divided into subtypes based on two proteins on the surface of the virus: Hemagglutinin (HA), of which there are 16 subtypes (H1-H16) Neuraminidase (NA), of which there are 9 subtypes (N1-N9) Many combinations of HA and NA proteins are possible (i.e., H5N1, H5N2, H7N2, H7N8, etc). [1][18] In other words, people exposed to a certain strain in childhood still possess antibodies to that strain at a reasonable level later in life, which can provide some protection to related strains. Asymptomatic infections are common. The A-type can also spread easily from one person to another. [11], Newly synthesized viral polymerase subunits and NP proteins are imported to the nucleus to further increase the rate of viral replication and form RNPs. [1][9], Antigenic shift is a sudden, drastic change in an influenza virus's antigen, usually HA. [4] These drugs are only functional against IAV[32] but are no longer recommended for use because of widespread resistance to them among IAVs. [23] By the end of 1920, it is estimated that about a third[11] to half of all people in the world had been infected, with tens of millions of deaths, disproportionately young adults. In 2009, a strain of flu virus known as H1N1 caused a respiratory infection in humans which spread rapidly around the world and became a pandemic. CDC twenty four seven. Most people with the flu get better on their own. [1], Equine IAVs include H7N7 and two lineages[1] of H3N8. information highlighted below and resubmit the form. [1][3] Similar to other aspects of influenza replication, optimal NA activity is temperature- and pH-dependent. [1][3], Progenic viruses leave the cell by budding from the cell membrane, which is initiated by the accumulation of M1 proteins at the cytoplasmic side of the membrane. H7N7, however, has not been detected in horses since the late 1970s,[19] so it may have become extinct in horses. [13], The pathophysiology of influenza is significantly influenced by which receptors influenza viruses bind to during entry into cells. [8] Adults who develop organ failure from influenza and children who have PIM scores and acute renal failure have higher rates of mortality. Advertising revenue supports our not-for-profit mission. Most people who get the flu can treat themselves at home and often don't need to see a health care provider. [1], The word influenza comes from the Italian word influenza, from medieval Latin influentia, originally meaning "visitation" or "influence". [1][21], B cells, a type of white blood cell, produce antibodies that bind to influenza antigens HA and NA (or HEF[18]) and other proteins to a lesser degree. [17], During the Second World War, the US government worked on developing inactivated vaccines for influenza, resulting in the first influenza vaccine being licensed in 1945 in the United States. In particular, reassortment is very common in AIVs, creating a large diversity of influenza viruses in birds, but is uncommon in human, equine, and canine lineages. Influenza A viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease. McGraw Hill; 2022. https://accessmedicine.mhmedical.com. Influenza. [39], Viruses can be grown in a culture of mammalian cells or embryonated eggs for 310 days to monitor cytopathic effect. Influenza, commonly known as " the flu ", is an infectious disease caused by influenza viruses. [1] From 1700 to 1889, influenza pandemics occurred about once every 5060 years. [24] Instead, most transmission involves domestic poultry, mainly chickens, ducks, and geese but also a variety of other birds such as guinea fowl, partridge, pheasants, and quails. [11] Baloxavir marboxil is a notable exception, which targets the endonuclease activity of the viral RNA polymerase and can be used as an alternative to NA and M2 inhibitors for IAV and IBV. Influenza (referred to as the "flu") is a contagious respiratory disease caused by the influenza A, B, or C virus. [13][25] Respiratory droplets are relatively large and travel less than two meters before falling onto nearby surfaces. [21] The high variability and rapid evolution of influenza virus antigens, however, is a major obstacle in developing effective vaccines. Seasonal flu is the type of influenza that typically causes illness for just a few months out of the year. AskMayoExpert. Consequently, the commonly recommended methods of infection control are respiratory etiquette, hand hygiene, and mask wearing, which are inexpensive and easy to perform. If you are a Mayo Clinic patient, this could MMWR Recommendations and Reports. The U.S. Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. Since the 2009 pandemic, oseltamivir resistance has mainly been observed in patients undergoing therapy,[1] especially the immunocompromised and young children. [3] After binding, the virus is internalized into the cell by an endosome that contains the virion inside it. This site is protected by reCAPTCHA and the Google The different subtypes and strains of influenza viruses are distinguished by the hemagglutinin and neuraminidase antigens they possess. [16] H3N8 in equines spreads via aerosols and causes respiratory illness. The endosome is acidified by cellular vATPase[20] to have lower pH, which triggers a conformational change in HA that allows fusion of the viral envelope with the endosomal membrane. These symptoms begin from one to four days after exposure to the virus (typically two days) and last for about 2-8 days. Since HA proteins vary in which sialic acids they bind to, mutations in the HA receptor binding site can allow AIVs to infect mammals. [11], Much research on LAIVs focuses on identifying genome sequences that can be deleted to create harmless influenza viruses in vaccines that still confer immunity. Colds usually develop slowly. Elsevier; 2020. https://www.clinicalkey.com. A small H5N1 outbreak in humans in Hong Kong occurred then,[30] and sporadic human cases have occurred since 1997, carrying a high case fatality rate. But the flu tends to come on suddenly. Get all the tips, tricks and info you need to protect yourself from Influenza before you start your trip. HA and HEF[18] proteins have a distinct "head" and "stalk" structure. [1] The seasonality of ICV, however, is poorly understood. There will also be a high-dose flu vaccine for adults age 65 and older. In long-term care facilities, the flu can spread rapidly after it is introduced. [33] Most flu vaccines are inactivated and administered via intramuscular injection. [39], Treatment of influenza in cases of mild or moderate illness is supportive and includes anti-fever medications such as acetaminophen and ibuprofen,[40] adequate fluid intake to avoid dehydration, and resting at home. Learn more about how to get your flu shot at Mayo Clinic, for video Mayo Clinic Minute: Why getting vaccinated for the flu is doubly important this season, Mayo Clinic on Incontinence - Mayo Clinic Press, NEW The Essential Diabetes Book - Mayo Clinic Press, NEW Mayo Clinic on Hearing and Balance - Mayo Clinic Press, FREE Mayo Clinic Diet Assessment - Mayo Clinic Press, Mayo Clinic Health Letter - FREE book - Mayo Clinic Press, Mayo Clinic Graduate School of Biomedical Sciences, Mayo Clinic School of Continuous Professional Development, Mayo Clinic School of Graduate Medical Education. Interferons mediate expression of antiviral proteins and proteins that recruit immune cells to the infection site, and they also notify nearby uninfected cells of infection. Influenza viruses are classified into 4 types: A, B, C, and D. Only virus types A and B commonly cause illness in humans. [1][21][3] PB1-F2 aso interacts with PB1 to keep polymerases in the nucleus longer. It is caused by an entirely different virus known as the human parainfluenza virus (HPIV). Goldman L, et al., eds. The sialidase activity of NA also cleaves any sialic acid residues from the viral surface, which helps prevent newly assembled viruses from aggregating near the cell surface and improving infectivity. HA is the main component of inactivated vaccines, so surveillance monitors antigenic drift of this antigen among circulating strains. [1] Samples may be taken from the lower respiratory tract if infection has cleared the upper but not lower respiratory tract. There are four different types of influenza virus: A, B, C, and D. Influenza C usually causes only mild illness while D mostly affects animals, especially cattle. This virus consists of three types: A, B, and C. Influenza types A and B has eight segments of RNA, but the influenza virus type C only has 7. To provide you with the most relevant and helpful information, and understand which This causes delays and changes to the signals being transmitted between the nerves and the brain. Most people will get better within 7 to 10 days. Equine (horse) influenza A(H3N8) virus routinely circulates and can cause illness in horses, and canine (dog) influenza A(H3N2) virus routinely circulates and can cause illness in dogs. [59][60][61] The shortened form of the word, "(the) flu", is first attested in 1839 as flue with the spelling flu first attested in 1893. But sometimes, influenza and its complications can be deadly. [22] At least one person, an elderly woman in Jiangxi Province, China, died of pneumonia in December 2013 from the H10N8 strain. This was the first pandemic to spread significantly by air travel. Other than this, the standard treatment remains bed rest, ingestion of fluids, and the use of analgesic s to control fever. BRD is a concern for the cattle industry, so IDV's possible involvement in BRD has led to research on vaccines for cattle that can provide protection against IDV. Influenza is commonly called the flu, but it's not the same as stomach "flu" viruses that cause diarrhea and vomiting. Research conducted during the past several years has led to an increased . If we combine this information with your protected The exception are bat influenza-like viruses, which have an uncertain origin. [13] About a third of primary pneumonia cases are followed by secondary pneumonia, which is most frequently caused by the bacteria Streptococcus pneumoniae and Staphylococcus aureus. It occurs most often during the winter and easily spreads from person to person from coughing, sneezing and/or touching contaminated surfaces. [11] Ultimately, presence of large quantities of viral RNA in the cell triggers apoptosis, i.e. [13] Additionally, febrile seizures and Reye syndrome can occur, most commonly in children. An exacerbated immune response, however, can harm the host organism through a cytokine storm. Grohskopf LA, et al. Avian influenza A(H7) virus infection of humans have occurred sporadically. diabetes) disorders,[8][1][9] as are people who are immunocompromised from chemotherapy, asplenia, prolonged steroid treatment, splenic dysfunction, or HIV infection. Influenza A viruses are divided into subtypes on the basis of two proteins on the surface of the virus:hemagglutinin (HA) and neuraminidase (NA). [8][15][4], NA inhibitors target the enzymatic activity of NA receptors, mimicking the binding of sialic acid in the active site of NA on IAV and IBV virions[1] so that viral release from infected cells and the rate of viral replication are impaired. [3][23] H2N2 and H3N2 co-circulated after the pandemic until 1971 when H2N2 waned in prevalence and was completely replaced by H3N2. [23][45], A minor influenza pandemic occurred from 1847 to 1851 at the same time as the third cholera pandemic and was the first flu pandemic to occur with vital statistics being recorded, so influenza mortality was clearly recorded for the first time. While more than 100 different viruses can cause a cold, only influenza virus types A, B, and C cause the flu. [10][18], IDV has been isolated from pigs and cattle, the latter being the natural reservoir. Seasonal influenza epidemics caused by influenza A and B viruses result in 3 to 5 million severe cases and thousands of deaths . The latter referred to the disease's cause, which at the time was ascribed by some to unfavorable astrological conditions. The 1918 flu pandemic was also caused by an influenza A H1N1 virus. This includes frequent hand washing with soap and water, using alcohol-based hand sanitizers, and not touching one's eyes, nose, and mouth with one's hands. Preventing the flu and lowering the number of people with severe flu and complications could also lower the number of people needing to stay in the hospital. Most people who have flu are infected with the influenza type A virus. There are actually four different types of influenza viruses: influenza A, B, C, and D. Influenza A and B are the two types of influenza that cause epidemic seasonal infections nearly.

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