Onset is sudden with fever, rhinorrhea, sore throat and headache. Severe myalgias coupled with respiratory tract symptoms are typical of influenza [7]. Patients may also have red, watery eyes and cough. There can also be tachycardia and dyspnea but with hypotension. Vomiting and diarrhea may also occur, but are rare findings associated with the disorder.

• Cervical Lymphadenopathy

  Signs and symptoms Signs and symptoms of pediatric seasonal influenza include the following: High fever Chills Myalgia Headache Fatigue Sore throat/pharyngitis Nasal congestion Rhinitis Nonproductive cough Cervical lymphadenopathy Conjunctivitis Conjunctivitis [emedicine.medscape.com]

• Fever

  Individuals with influenza were more likely than those without influenza to have baseline cough (93% vs 80%), fever (68% vs 40%), and cough and fever together (64% vs 33%). [archinte.ama-assn.org]

  Individuals with influenza were more likely to have cough (93% vs 80%), fever (68% vs 40%), cough and fever together (64% vs 33%), and/or nasal congestion (91% vs 81%) than those without influenza. [ncbi.nlm.nih.gov]

  Keep your child at home, except for doctor visits, for at least 24 hours after their fever is gone without the use of fever-reducing medicines. Make sure your child gets plenty of rest and clear fluids (water, broth, sports drinks, etc.). [cdc.gov]

• Fatigue

  An 88-year-old woman presented to our emergency room with complaints of fever, coryza, barking cough and generalised fatigue for 2 days. Physical examination showed stridor, tachypnoea with use of accessory muscles of respiration on admission. [ncbi.nlm.nih.gov]

  Infected individuals usually experience a rapid onset of flu symptoms which typically include: muscle aches, high fever, head ache, chills, fatigue, dry cough and runny/stuffy nose. [3dscience.com]

  People who have flu often feel some or all of these symptoms: fever* cough sore throat runny or stuffy nose body aches headache chills fatigue sometimes diarrhea and vomiting *It’s important to note that not everyone with flu will have a fever. [cdc.gov]

  These symptoms can vary from mild to severe and often include the following: • A fever • Runny nose • Sore throat • Headaches • Cough • Fatigue • Body aches • Diarrhoea and vomiting How to Tell If Your Child Has H1N1 It may be difficult to tell if your [healthhub.sg]

  […] flu·en·za \ ˌin-(ˌ)flü-ˈen-zə \ 1 a : an acute, highly contagious, respiratory disease caused by any of three orthomyxoviruses: (1) or influenza A : moderate to severe influenza that in humans is marked especially by sudden onset, fever, sore throat, fatigue [merriam-webster.com]

• Chills

  They may include body or muscle aches chills cough fever headache sore throat is it a cold or the flu? colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. [icd9data.com]

  Moderate to high fever Dry cough Headache Sore throat Chills Runny nose Loss of appetite Muscle aches Tiredness Many conditions — such as a common cold, diarrhea, and vomiting — are called "the flu," but are not influenza. [my.clevelandclinic.org]

  […] studies limited to patients aged 60 years or older, the combination of fever, cough, and acute onset (LR, 5.4; 95% CI, 3.8-7.7), fever and cough (LR, 5.0; 95% CI, 3.5-6.9), fever alone (LR, 3.8; 95% CI, 2.8-5.0), malaise (LR, 2.6; 95% CI, 2.2-3.1), and chills [ncbi.nlm.nih.gov]

  It is marked by inflammation of the nasal mucosa, pharynx, and conjunctiva; headache; myalgia; often fever, chills, and prostration; and occasionally involvement of the myocardium or central nervous system. [fpnotebook.com]

• Weakness

  Despite treatment using intravenous immunoglobulin, her condition continued to get worse with new bulbar and respiratory muscle weakness. [ncbi.nlm.nih.gov]

  Among those individuals without influenza, weakness (94%) and myalgia (94%) were also commonly reported. [archinte.ama-assn.org]

  In older adults, feeling weak and confused sometimes may be the only symptoms. The flu is often confused with other respiratory infections such as the cold. To learn more, go to the page Differences between a Cold and the Flu. [quebec.ca]

• High Fever

  It is a syndrome encompassing an onset of high fever, disturbance of consciousness, convulsion, and shock that rapidly progresses to watery diarrhea and liver and renal dysfunctions. [ncbi.nlm.nih.gov]

  Infection usually lasts for about a week, and is characterized by sudden onset of high fever, aching muscles, headache and severe malaise, non-productive cough, sore throat and rhinitis. [paho.org]

  The most common symptoms include: a high fever, runny nose, sore throat, muscle pains, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. [imedpub.com]

  It is characterized by sudden onset of high fever, myalgia, headache and severe malaise, nonproductive cough, sore throat, and rhinitis. Most people recover within one to two weeks without requiring any medical treatment. [certest.es]

  Moderate to high fever Dry cough Headache Sore throat Chills Runny nose Loss of appetite Muscle aches Tiredness Many conditions — such as a common cold, diarrhea, and vomiting — are called "the flu," but are not influenza. [my.clevelandclinic.org]

• Cough

  Individuals with influenza were more likely to have cough (93% vs 80%), fever (68% vs 40%), cough and fever together (64% vs 33%), and/or nasal congestion (91% vs 81%) than those without influenza. [ncbi.nlm.nih.gov]

  Individuals with influenza were more likely than those without influenza to have baseline cough (93% vs 80%), fever (68% vs 40%), and cough and fever together (64% vs 33%). [archinte.ama-assn.org]

  Health-care Settings Symptomatic or Infected Patients During periods of increased respiratory infection activity in the community, masks should be offered as part of a respiratory hygiene/cough etiquette strategy to patients who are coughing or have other [web.archive.org]

• Sneezing

  How Flu Spreads Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby. [cdc.gov]

  The mechanisms of symptoms of sore throat, rhinorrhoea, sneezing, nasal congestion, cough, watery eyes, and sinus pain are discussed, since these mechanisms are not dealt with in any detail in standard medical textbooks. [ncbi.nlm.nih.gov]

  How it spreads The flu spreads: when an infected person coughs or sneezes, and you breathe it in through direct contact with fluid from an infected person’s coughs or sneezes by touching a contaminated surface with the flu virus on it, and then touching [health.gov.au]

  Yet differing symptoms of the cold are: sore throat; productive, phlegmy coughs; and sneezing. [3dscience.com]

• Respiratory Distress

  To present a case of influenza A infection complicated with focal encephalitis, meningitis, and acute respiratory distress syndrome. A 35-year-old woman presented with fever, headache, cough, and body aches. [ncbi.nlm.nih.gov]

  A Fatal Case of Acute Respiratory Distress Syndrome. [pnas.org]

• Nasal Congestion

  The mechanisms of symptoms of sore throat, rhinorrhoea, sneezing, nasal congestion, cough, watery eyes, and sinus pain are discussed, since these mechanisms are not dealt with in any detail in standard medical textbooks. [ncbi.nlm.nih.gov]

  For decreasing the likelihood of influenza, the absence of fever (LR, 0.40; 95% confidence interval [CI], 0.25-0.66), cough (LR, 0.42; 95% CI, 0.31-0.57), or nasal congestion (LR, 0.49; 95% CI, 0.42-0.59) were the only findings that had summary LRs less [dx.doi.org]

• Dry Cough

  The dry cough and fatigue of influenza can last two to three weeks. Signs that influenza is getting worse include fever, shaking chills, and shortness of breath. [my.clevelandclinic.org]

  Common symptoms are fever, chills, headache, tiredness, sore throat, dry cough and runny or stuffy nose. [straitstimes.com]

  Infected individuals usually experience a rapid onset of flu symptoms which typically include: muscle aches, high fever, head ache, chills, fatigue, dry cough and runny/stuffy nose. [3dscience.com]

  A bout of the flu typically follows this pattern: Days 1–3: Sudden appearance of fever, headache, muscle pain and weakness, dry cough, sore throat and sometimes a stuffy nose. Day 4: Fever and muscle aches decrease. [betterhealth.vic.gov.au]

  Moderate to severe illness Mild illness Fever (usually high) Mild fever Headache (may be severe) Mild headache (congested sinuses) Dry cough, may become moist Sometimes a cough Muscle aches Muscle aches are uncommon Shivering A runny nose Best rest necessary [fightflu.co.nz]

• Nausea

  In children, there may be nausea and vomiting, but these are not common in adults. [imedpub.com]

  These data, shown in Table 2, show strong evidence of increased incidence of nausea and vomiting due to oseltamivir. Open in table viewer Table 1. [dx.doi.org]

  In oseltamivir trials, nausea, vomiting and diarrhea were significant side-effects. For zanamivir trials, no adverse effects have been reported. [ncbi.nlm.nih.gov]

  Flu signs and symptoms usually include fever and respiratory symptoms, such as cough and runny nose, and possibly other symptoms, such as body aches, nausea, vomiting, or diarrhea. [chicago.gov]

  The flu in children In addition to fever and respiratory symptoms such as a runny nose and cough, cases involving abdominal symptoms such as nausea, stomach ache, and diarrhea, or neurological symptoms such as spasms and disorders of consciousness, as [otsuka.co.jp]

• Abdominal Pain

  Abdominal pain, often severe enough to require differentiation from acute appendicitis, emerged as a dominant symptom, especially in older children. [ncbi.nlm.nih.gov]

  The main symptoms are as follows: Sudden fever, between 39 C and 40 C (102 F and 104 F) Sudden cough Sore throat Muscle or joint pain Extreme fatigue Headaches Symptoms can also include nausea, vomiting, diarrhea and abdominal pain. [quebec.ca]

  Influenza A symptoms If you have influenza, you will have some or all of these symptoms: fever and chills headache and muscle aches feeling tired and weak sneezing, and stuffy or runny nose sore throat and cough Children may also have abdominal pain, [healthdirect.gov.au]

• Loss of Appetite

  Moderate to high fever Dry cough Headache Sore throat Chills Runny nose Loss of appetite Muscle aches Tiredness Many conditions — such as a common cold, diarrhea, and vomiting — are called "the flu," but are not influenza. [my.clevelandclinic.org]

  Usually they include the sudden appearance of: fever cough muscle aches and pain Other common symptoms may include: headache chills fatigue (tiredness) loss of appetite sore throat runny or stuffy nose Some people (especially children) may also have: [canada.ca]

  Other common symptoms include sore throat, runny nose, headache, chills, loss of appetite, and fatigue. Some adults will also experience stomach ache, vomiting and diarrhea, but these symptoms are more common with children. [ccohs.ca]

  Data collection and analysis The following baseline signs or symptoms were analyzed as potential predictors of influenza infection: fever, feverishness, cough, headache, sore throat, myalgia, nasal congestion, weakness, and loss of appetite. [archinte.ama-assn.org]

• Chest Pain

  We suggest considering recent immunization in patient's history as part of the differential diagnosis in elderly with chest pain. [ncbi.nlm.nih.gov]

  Immediate consultation at an emergency room You must go to the emergency room immediately if you have flu symptoms and also one of the following symptoms: Persistent or increasing difficulty breathing Blue lips Severe chest pain Persistent or increasing [quebec.ca]

  You should also get help straight away if you feel chest pain, short of breath, dizzy or confused, or you are vomiting a lot. [healthdirect.gov.au]

• Tachycardia

  The next morning, he complained of acute-onset shortness of breath and had hypotension and tachycardia. On examination, his peripheral extremities were cold and heart sounds were distant. Pulsus paradoxus was 20 mmHg. [ncbi.nlm.nih.gov]

  Typical signs and symptoms include the following (not necessarily in order of prevalence): Cough and other respiratory symptoms Fever Sore throat Myalgias Headache Nasal discharge Weakness and severe fatigue Tachycardia Red, watery eyes Cough and other [emedicine.medscape.com]

  There can also be tachycardia and dyspnea but with hypotension. Vomiting and diarrhea may also occur, but are rare findings associated with the disorder. [symptoma.com]

  […] presentation of influenza virus infection varies, but it usually includes many of the following signs and symptoms: Fever Sore throat Myalgias Frontal or retro-orbital headache Nasal discharge Weakness and severe fatigue Cough and other respiratory symptoms Tachycardia [emedicine.com]

• Hypotension

  The next morning, he complained of acute-onset shortness of breath and had hypotension and tachycardia. On examination, his peripheral extremities were cold and heart sounds were distant. Pulsus paradoxus was 20 mmHg. [ncbi.nlm.nih.gov]

  There can also be tachycardia and dyspnea but with hypotension. Vomiting and diarrhea may also occur, but are rare findings associated with the disorder. [symptoma.com]

  Only one immediate reaction was reported: one subject vaccinated with Agrippal S1 â experienced increased sweating and hypotension within the 30 minutes of vaccination. [scielo.org.ar]

  Management Most hospitalized patients with avian influenza A (H5N1) have required ventilatory support within 48 hours after admission, 15,16 as well as intensive care for multiorgan failure and sometimes hypotension. [doi.org]

• Flushing

  The skin may be warm and flushed, especially on the face. The mouth and throat may redden, the eyes may water, and the whites of the eyes may become bloodshot. People, especially children, may have nausea and vomiting. [web.archive.org]

  Other common symptoms include: Body aches Chills Dizziness Flushed face Headache Lack of energy Nausea and vomiting The fever, aches, and pains begin to go away on days 2 through 4. [medlineplus.gov]

• Increased Sweating

  Only one immediate reaction was reported: one subject vaccinated with Agrippal S1 â experienced increased sweating and hypotension within the 30 minutes of vaccination. [scielo.org.ar]

• Photophobia

  Headache is prominent, often with photophobia and retrobulbar aching. Respiratory symptoms may be mild at first, with scratchy sore throat, substernal burning, nonproductive cough, and sometimes coryza. [msdmanuals.com]

  Red eyes were reported most frequently (94%) in cases of conjunctivitis, followed by tearful (77%), burning (64%), and itching (57%) eyes, and by pus in eyes (48%), photophobia (33%), and pain in eyes (31%). [pnas.org]

  Ocular symptoms develop in some patients with influenza and include photophobia, burning sensations, or pain upon motion. Some patients with influenza develop rhinitis of varying severity, but it is generally not the chief symptom. [emedicine.medscape.com]

• Blepharitis

  He was a smoker, had mild recurrent labial herpes, and was treated for glaucoma, a recurrent mild allergic blepharitis, and conjunctivitis. He did not receive an influenza vaccination. [pnas.org]

• Myalgia

  It is marked by inflammation of the nasal mucosa; the pharynx; and conjunctiva, and by headache and severe, often generalized, myalgia. [icd9data.com]

  The influenza A virus has accounted for the majority of influenza infections in the 2017 to 2018 flu season, with the typical clinical presentation including fever, myalgias, malaise, and nonproductive cough. [ncbi.nlm.nih.gov]

  Among those individuals without influenza, weakness (94%) and myalgia (94%) were also commonly reported. [archinte.ama-assn.org]

• Arthralgia

  This is a case of a 60-year-old man with chronic idiopathic pulmonary fibrosis, with fever, arthralgias and rash characterised by extensive erythematous plaques on his limbs and trunk 5â ‰days after influenza vaccination. [ncbi.nlm.nih.gov]

  Common treatments include heat, rest, paracetamol, NSAIDs and muscle relaxants.[6] See also[edit] Myopathy Myositis Myelitis Arthralgia References[edit] External links[edit] Classification D ICD-10: M79.1 ICD-9-CM: 729.1 MeSH: D063806 DiseasesDB: 22895 [en.wikipedia.org]

  We defined RIDT-negative influenza-like illness as RIDT-negative cases suspected by doctors due to clinical signs such as fever, headache, arthralgia, runny nose, and/or coughing as well as close contact with patients with influenza. [ajcn.org]

• Back Pain

  (n 479) n (%) P‐value (Fishers exact) Vomiting 15 (3.2) 57 (11.9) Nausea 29 (6.2) 70 (14.6) Insomnia 3 (0.6) 7 (1.5) 0.34 Constipation 1 (0.2) 4 (0.8) 0.37 Back pain 2 (0.4) 4 (0.8) 0.69 Type of dizziness 2 (0.4) 4 (0.8) 0.69 Headache 11 (2.4) 13 (2.7 [dx.doi.org]

• Epistaxis

  APARATO RESPIRATORIO R01 Dolor atribuido al aparato respiratorio R02 Fatiga respiratoria/disnea R03 Respiración jadeante/sibilante R04 Otros problemas de la respiración R05 Tos R06 Epistaxis/hemorragia nasal R07 Estornudos/congestión nasal R08 Otros signos [iqb.es]

  La diarrea acuosa, vómito, dolor abdominal y pleural, así como epistaxis y gingivorragia son manifestaciones comunes en los estados iniciales de la enfermedad. 19,163 Algunos pacientes pueden exhibir compromiso del sistema nervioso central. [scielo.org.mx]

• Headache

  They may include body or muscle aches chills cough fever headache sore throat is it a cold or the flu? colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. [icd9data.com]

  A 35-year-old woman presented with fever, headache, cough, and body aches. Seizures, altered consciousness, and dyspnea occurred later. A nasopharyngeal swab revealed a positive reaction for the influenza A antigen. [ncbi.nlm.nih.gov]

  They may include Body or muscle aches Chills Cough Fever Headache Sore throat Is it a cold or the flu? Colds rarely cause a fever or headaches. Flu almost never causes an upset stomach. And "stomach flu" isn't really flu at all, but gastroenteritis. [fpnotebook.com]

• Guillain-Barré Syndrome

  An increased risk of Guillain-Barré syndrome was seen in the period shortly after influenzalike illness, consistent with observations that Guillain-Barré syndrome is often preceded by a respiratory illness. [doi.org]

  Cerebral Spinal Fluid study showed normal cell counts with elevated protein and nerve conduction study showed reduced diffuse compound muscle action potential amplitudes suggesting a diagnosis of Guillain-Barré syndrome. [ncbi.nlm.nih.gov]

  At the time, doctors thought the vaccine triggered Guillain-Barré syndrome, but more recent evidence makes this connection uncertain. [merck.com]

• Febrile Seizures

  The risk of febrile seizures when IIV is administered with other vaccines to children 6 through 23 months of age needs further investigation. [pediatrics.aappublications.org]

  Causality could not be proved, and in some studies the seizures were thought to be febrile seizures. [web.archive.org]

  Young children with influenza infection can have initial symptoms mimicking bacterial sepsis with high fevers ( 41,42 ), and 20% of children hospitalized with influenza can have febrile seizures ( 32,43 ). [cdc.gov]

  One child in every 110 aged below five who was vaccinated with the CSL vaccine had a febrile seizure. Australia consequently suspended its use. [doi.org]

Laboratory tests

• Complete blood count
• Nasal or throat wash
• Nasal or throat swab
• PCR
• Cell-culture based tests
• FLU OIA test and QUICKVUE Influenza test based on detection of viral antigens by using monoclonal antibodies [7].
• ZSTATFLU test based on detection of viral neuraminidase using a substrate of colour-changing enzyme.
• Alere I Influenza A & B test a new point-of-care influenza test that delivers highly accurate molecular results in less than 15 minutes, approved by FDA in June 2014.

Imaging 

Imaging studies are generally not required. In case of complications such as bacterial pneumonia, chest radiography may be advised.

Test results

A clinical examination and history is sufficient to form a diagnosis. Laboratory tests can help confirm the diagnosis.

Pharmacological treatment

Lam et al suggested that cases of suspected severe influenza infection should be treated early and aggressively, even before diagnostic tests can be confirmed [8]. Commonly used antivirals include neuraminidase inhibitors like oseltamivir and zanamivir. M2 protein ion channel inhibitors, which are adamantine derivatives, like amantidine and rimantidine can also be used. Due to a marked increase in resistant isolates, the Advisory Committee on Immunization Practices (ACIP) recommends that adamantanes not be used in the United States for the treatment of influenza, except in selected circumstances [9].

Non-pharmacological treatment

It includes bed rest, the increased intake of fluids and a light but healthy diet during the flu. Typically, flu will resolve within a few days without any treatment.

Influenza has an excellent prognosis. Like all infectious diseases, it has a certain duration of action after which the symptoms begin to slowly subside. When there are no complications, acute symptoms subside within 3-7 days. Post-influenza asthenia may last for a few weeks.

The mortality associated with influenza is due to the complications associated with the disease. As a result of these complications, influenza can be deadly, especially for the weak, young and old, or chronically ill [6]. They include bacterial pneumonia, bronchitis, myositis, Reye syndrome in children, myocarditis and in some rare cases, multiorgan failure.

Causative factors

There are 3 basic causative factors of this disease: Influenza A, B and C virus. They belong to the Orthomyxovirus family. Of the three, Influenza A virus is considered to be the most pathogenic and has at least 4 different recognized strains.

Transmission

Although the respiratory tract is presumed to be the primary site for the acquisition of infection, one study has suggested that transocular entry of influenza virus can occur [2]. Infection can be transmitted through sneezing and coughing and is thought to be transmitted primarily via large particle droplets (>5 microns) [3]. It is an airborne infection and close contact with a person with an active infection can transmit the disease.

Incidence

According to a recent estimate by World Health Organization, 3 to 5 million cases of severe cases occur worldwide annually. 250,000 to 500,000 deaths have also been reported [4].

Age

Influenza affects young and old alike. Elderly patients with low immunity and immunocompromised individuals are particularly susceptible.

Sex

Influenza occurs equally in both sexes.

Influenza A is a genetically labile virus with mutation rates as high as 300 times that of other microbes [5]. It is known to cause pandemics worldwide. Influenza B is usually associated with smaller outbreaks. Influenza C has been reported to cause epidemics in children but mainly causes only mild respiratory infections.

After the virus has been inhaled, an incubation period of 12-24 hours ensues. A special substance present in the form of spikes on the virus is hemagglutinin. It binds to cell surface receptors to initiate infection. Another substance, neuroaminidase, present in the virus is an enzyme that degrades the protective layer of mucus in the respiratory tract. It also cleaves neuraminic acid to release progeny virus from the infected cells. It is the action of these two moieties that result in the symptoms of influenza as well as necrosis of the superficial layers of the respiratory epithelium.

The main mode of prevention is the influenza vaccine that typically contains two strains of Influenza A virus and one strain of Influenza B virus. Annual influenza vaccination is an important public health measure for preventing influenza infection [10]. Currently, there are two types of vaccines available in the US. First is a killed vaccine containing purified protein subunits of the virus (hemagglutinin and neuraminidase). It has two versions, namely ‘split virus’ vaccine and ‘purified subunit’ vaccine. Both are administered intramuscularly. The second type is a live vaccine which contains temperature sensitive mutants of Influenza A and B virus. 

Some antiviral drugs may also help in preventing the infection, particularly in elderly, non-vaccinated individuals.

Influenza is a common household occurrence. Commonly known as the ‘flu’, influenza is an infectious disease caused by a virus. Although it is often confused with other influenza-like illnesses, especially the common cold, influenza is a more severe disease [1]. It has been known to cause epidemics and pandemics worldwide. Influenza has a seasonal variation, being much more common in winter and in the rainy season. It has variations which pose to be a much bigger threat like the avian flu and the swine flu, however, our section deals with the general concept of this disease.

Definition

Influenza, simply called the flu, is a very common infectious disease of viral origin.

Cause

It is caused by Influenza virus A, B or C. Transmission is through airborne aerosol droplets that contain the infectious particles. It can be spread by close contact with a person having an active infection. Simple acts such as coughing, sneezing and even laughing can transmit the infection.

Signs and symptoms

Signs and symptoms include sudden high grade fever associated with chills, sore throat, runny nose and cough. Headache and body aches are commonly present. There may also be difficulty in breathing, increased heart rate, vomiting and diarrhea. Symptoms may go away on their own eventually within 3-7 days.

Diagnosis

Diagnosis is made based on clinical findings. Some laboratory tests may be performed to confirm the diagnosis. In case of complications such as pneumonia, a more through series of tests will be required.

Treatment

Treatment is not necessary as symptoms often resolve by themselves. Bed rest and fluids help speed up recovery. Some antiviral drugs may be used if needed along with painkillers to relieve headaches and muscle pain.

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2. Bischoff WE, Reid T, Russell GB, Peters TR. Transocular entry of seasonal influenza-attenuated virus aerosols and the efficacy of n95 respirators, surgical masks, and eye protection in humans. J Infect Dis 2011; 204:193.
3. Mubareka S, Lowen AC, Steel J, et al. Transmission of influenza virus via aerosols and fomites in the guinea pig model. J Infect Dis 2009; 199:858. 
4. Lozano R. Global and regional mortality from 235 causes of death for 20 age groups in 1990 and 2010: a systematic analysis for the Global Burden of Disease Study 2010. Lancet 380 (9859): 2095–128, 2012
5. Drake JW. Rates of spontaneous mutation among RNA viruses. Proc Natl Acad Sci U S A. May 1 1993;90(9):4171-5.
6. Hilleman M. Realities and enigmas of human viral influenza: pathogenesis, epidemiology and control. Vaccine. Aug 2002. 20 (25–26): 3068–87. 
7. Warren L. Review of Medical Microbiology and Immunology. 11th ed. Pa: McGraw Hill, 2010. ISBN 978-0-07-170028-3. 
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9. Fiore AE, Fry A, Shay D, et al. Antiviral agents for the treatment and chemoprophylaxis of influenza --- recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep 2011; 60:1. 
10. Grohskopf LA, Olsen SJ, Sokolow LZ, et al. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP) -- United States, 2014-15 influenza season. MMWR Morb Mortal Wkly Rep 2014; 63:691.