Influenza is a contagious viral disease of the airways

Influenza (ICD-9 code 487,ICD-10 code J10-11), often also called the flu, is an infectious disease of the airways. It is caused by infection with RNA viruses of the family Orthomyxiviridae. These viruses can infect both birds and mammals. The infection spreads a.o. through droplets (aerosols) that originate from coughing and sneezing (Tellier, 2006; Weber & Stilianakis, 2008).

Seasonal outbreaks occur especially during the cold seasons. The incubation period of influenza is one to four days. During the first week of the disease it is contagious in close contact. Outbreaks in humans are caused by influenza viruses type A or B (Nicholson et al., 2003).

Influenza begins with acute fever

The disease often begins with fever and chills, accompanied by headache and sore throat, myalgias, malaise, anorexia, and a dry cough. Fever (38–40°C) peaks within 24 h of onset and lasts 1–5 days. Other symptoms include nausea, abdominal pain, and diarrhea (Betts, 1995, Douglas, 1975, Nicholson, 1992). In children, symptoms are similar to those in adults; however, gastrointestinal problems, such as vomiting, abdominal pain, and diarrhea, are more frequent. Maximum temperatures also tend to be higher in children than in adults, and febrile convulsions can occur (Price et al., 1976). The patients can still feel not fit few days or weeks after the fever is gone. Healthy people overcome influenza within 1 to 2 weeks without sequelae (Cox & Fukuda, 1998, Cox & Subbarao, 1999).

Influenza sometimes associated with severe complications

Influenza may be associated with the following complications (CDC, 2009a):

• Pneumonia (usually additional bacterial infections)
• Worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.
• Acute otitis media and sinus problems in children

Those aged 65 years and older and persons of any age with chronic medical conditions, i.e. asthma, diabetes, or heart disease, are at highest risk for serious complications. Therefore these groups are in many countries advised to be vaccinated. See also:Causes and risk factors.

Respiratory infections are not always influenza

On clinical grounds it is difficult to make a clear distinction between infections caused by influenza viruses and those caused by other viruses or some bacteria. In practice, the diagnosis stated is often “influenza-like illness” (ILI) or ”acute respiratory infection” (ARI). For more information see European Union definition of influenza. Confirmation of the presence of the virus with a laboratory test may confirm a case of ILI to be a case of influenza.

Almost every year new virus variants appear

The influenza virus family has 3 types: A B and C. Only the A and B types cause widespread outbreaks of flu among humans (Nicholson et al., 2003). Type A is more important than type B in terms of disease severity and mortality. The type A viruses are characterized by a high degree of variation in two of their surface antigens, the so-called haemagglutinins (HA; subtypes H1-H16) and neuraminidases (NA; subtypes N1-N9).

From time to time new virus variants emerge due to genetic changes in the virus that cause alterations in the antigens on the surface of the virus (antigenic drift). Because the human body defends against surface antigens, the virus in this way partially circumvents the body-immunity that was build up by a previous natural infection or vaccination. Therefore, the influenza virus is able to cause a new epidemic almost every year.

There is also a possibility that a completely new subtype of influenza A virus will emerge (antigenic shift). Since it is completely new, people are naturally not immune protected against it at all, and therefore such a virus variant can lead to a widespread influenza pandemic.

Epidemics and pandemics of influenza among humans since 1889 have been caused by viruses with haemagglutinins of the H1, H2, and H3 subtypes (Cox & Fukuda, 1998). These subtypes can circulate simultaneously during an epidemic and simultaneously with influenza B viruses as well.

See also What is an influenza pandemic?