Influenza A (H1N1), A (H3N2), and influenza B viruses currently circulate among humans worldwide. The main two types of influenza are seasonal and zoonotic. The best way to prevent influenza is annual flu vaccination.

37,500,000 (US); 750,000,000 (Global)
Influenza virus infection, types A and B; Seasonal influenza (H3N2, H1N1, and B); Avian (H5N1);
Average 2 days (1-4 days) for seasonal influenza; May be up to one week or longer for avian viruses; [Cecil, p. 2157]
Flu-like illness with respiratory symptoms; GI symptoms are uncommon in adults, but occur in up to 25% of children in school outbreaks; [CCDM]
Droplet for 5 days; For duration of illness in immunocompromised persons. "Single patient room when available or cohort; avoid placement with high-risk patients; mask patient when transported out of room; chemoprophylaxis/vaccine to control/prevent outbreaks. Use gown and gloves according to Standard Precautions may be especially important in pediatric settings. Duration of precautions for immunocompromised patients cannot be defined; prolonged duration of viral shedding (i.e. for several weeks) has been observed; implications for transmission are unknown." [CDC 2007 Guideline for Isolation Precautions] See for current avian influenza guidance. Annual immunization is recommended for healthcare workers. See
Influenza A (H1N1), A (H3N2), and influenza B viruses currently circulate among humans worldwide. The main two types of influenza are seasonal and zoonotic. The best way to prevent influenza is annual flu vaccination. Avian flu carried by wild birds, especially water fowl, does not commonly infect humans. From 1997 through January 2018, about 860 humans were infected with avian influenza A (H5N1) virus. The H5N1 strain causes 53% mortality in humans and is acquired from close contact with sick or dead poultry. This strain is widespread among poultry in Asia and the Middle East. [CDC Travel, p. 239-48] Swine flu viruses do not usually infect humans. "In 2012, a multistate outbreak of influenza caused by a swine-origin (variant) influenza A (H3N2) virus occurred in the United States, with 307 recorded cases in 11 Midwestern and Middle Atlantic states." [PPID, 8th Ed, p. 167] See information on Avian Flu and Swine Flu. [] Influenza is spread by all routes (droplet, droplet nuclei, and contact). [CCDM, p. 309]

Arthralgia is common. Other symptoms are dry cough, sore throat, rhinorrhea, congestion, and slightly enlarged, tender cervical lymph nodes. Fever usually lasts for 3 days, sometimes 4-8 days. Children tend to have higher fevers and more prominent cervical lymphadenopathy. Some older patients present with high fever and confusion without respiratory symptoms. Primary influenza viral pneumonia is associated with rheumatic heart disease/mitral stenosis. Myocarditis and pericarditis are rare complications. [PPID, p. 2152-3] Mild conjunctivitis may occur. [Merck Manual, p. 1686] Influenza and other respiratory viruses are common causes of petechiae. [Harrison ID, 470t] Complications of influenza include viral pneumonia, bacterial pneumonia, encephalitis, Reye's syndrome (after aspirin use), and rhabdomyolysis (myoglobinuria) followed by acute renal failure. [ID, p. 1990-1] Biphasic fever is uncommon. Other complications are sepsis, thrombocytopenia, disseminated intravascular coagulation, aseptic meningitis, myelitis, and Guillain-Barre syndrome. Dyspnea, lung infiltrates, and bloody sputum are findings in primary influenza viral pneumonia. [Cecil, 24th Ed, p. 2098] Other complications are transverse myelitis and toxic shock syndrome. Sputum production is scanty in primary influenza viral pneumonia. [Harrison ID, p. 787]

The first outbreak of H5N1 (Bird) flu occurred in Hong Kong in 1997. "The current situation of widespread outbreaks of highly pathogenic avian influenza A (H5N1) virus infection among poultry is of great concern because the A (H5N1) virus is now endemic in poultry in some countries, causes high rates of death among infected poultry, and causes high case fatality in humans (60% of reported human cases have been fatal)." Patients present with severe pneumonia (fever, cough, and dyspnea). An atypical presentation is fever, diarrhea, seizures, coma and no pneumonia. Common laboratory abnormalities are leukopenia, thrombocytopenia, and elevated aminotransferases. [CCDM, p. 313-22] Poor prognostic signs in H5N1 are leukopenia, thrombocytopenia, and elevated liver enzymes. [PPID, p. 2150] "An outbreak of human infections with a new avian influenza A (H7N9) virus was first reported in China by the World Health Organization on April 1, 2013." [] Conjunctivitis is a characteristic symptom of avian H7 infection, but not in recent zoonotic H7N9 cases. [Cecil, p. 2097]

"Effectiveness of influenza vaccines varies from year to year and depends on the age and health status of the person getting the vaccine and the similarity of 'match' between the viruses or virus in the vaccine and those in circulation. . . Oseltamivir or zanamivir are recommended currently for chemoprophylaxis or treatment of influenza. . . . TIV [trivalent inactivated vaccine] can be administered to exposed, unvaccinated HCP at the same time as chemoprophylaxis, but LAIV [live attenuated influenza vaccine] should be avoided because the antiviral medication will prevent viral replication needed to stimulate a vaccine response. . . In many instances of HCP exposure, watchful waiting and early initiation of treatment if symptoms appear is preferred rather than use of antiviral chemoprophylaxis immediately after exposure." [ACIP, 2011]
Most sensitive are RT-PCR and viral culture. Compared to RT-PCR, other rapid tests lack sensitivity. Specialized lab tests for H5N1 when patient was exposed to poultry; [CCDM] See updated recommendations on diagnostic tests at
  • >arthralgia
  • >fatigue, weakness
  • >fever
  • >fever, biphasic or relapsing
  • >myalgia
  • E pharyngitis
  • E rhinitis
  • G abdominal pain
  • G diarrhea
  • G liver function test, abnormal
  • G nausea, vomiting
  • H leukopenia
  • H lymphadenopathy
  • H thrombocytopenia
  • N confusion, delirium
  • N headache
  • N lethargy
  • N seizure
  • O conjunctivitis, acute
  • R chest pain
  • R cough
  • R dyspnea
  • R hemoptysis
  • R sputum production
  • R wheezing
  • S petechiae and ecchymoses
  • X lung infiltrates
  • *acute renal failure
  • *ARDS
  • *bleeding tendency
  • *encephalitis
  • *meningitis
  • *myelitis
  • *myocarditis
  • *paralysis
  • *parotitis
  • *pericarditis
  • *pneumonia
  • *rhabdomyolysis
  • *sepsis
  • *shock
  • *stupor, coma




Inhalation, Skin or Mucous Membranes (Includes Conjunctiva)
Person-to-Person, Animal Excreta
Birds and Poultry, Swine, Human
  • Care for patients (droplet/airborne)
  • Fail to complete immunizations
  • Handle domestic animals
  • Handle infected chickens or birds
  • Live together in close quarters
  • Travel to endemic area
  • Victim--air release of infectious agents
See updated recommendations for the use of antiviral medications at
1. (US) 5% to 20% of population affected every year; 36,000 deaths every year; [Gorbach, p. 218] Calculate 12.5% X 300 million = 37.5 million;
2. (Global) 3-5 million cases of severe illness and 250,000 to 500,000 deaths/yr; Several hundred human deaths since 2003 from avian influenza; [Fact sheets from WHO 2013] 20 X US cases/yr;