Influenza is a contagious viral disease

Influenza viruses are spread primarily by small particle aerosols of virus-laden respiratory secretions that are expelled into the air by an infected person during coughing, sneezing, or talking (Tellier, 2006; Weber & Stilianakis, 2008). Spread by direct contact may also occur. Influenza epidemics and pandemics often occur explosively with simultaneous onset of illness in many persons within a relatively short period of time. This occurs because the incubation period for influenza is short (1-4 days) and a single infected person can transmit virus to a large number of susceptible individuals (Cox & Fukuda, 1998).

Risk groups for influenza

Some groups of patients are at increased risk for severe complications and deaths due to influenza (WHO, 2009f). These include:

• Residents of institutions for elderly people and the disabled;
• People of any age with certain chronic health conditions (such as chronic heart or lung disease, metabolic or renal disease or immunodeficiencies);
• Elderly people
• Very young children.

Influenza as a special risk to the elderly

The risk of complications and mortality from influenza increases significantly in elderly. Elderly people have an increased risk of a permanent reduction in their physical and mental capabilities (Sprenger et al., 1993b). In some cases their death is attributed to secondary bacterial pneumonia, but sometimes death occurs without any apparent complication. Relatively many influenza patients have an abnormal ECG (Karjalainen et al., 1980; Greaves et al., 2003). This symptom can be fatal even after long time especially in chronic heart patients. Influenza patients may develop several types of encephalopathy (brain disorders) (Mizuguchi et al., 2007). Usually it heals quickly and completely. Sometimes, however, brain haemorrhage arises that could have lasting repercussions. Approximately 20% of children 6 months to 5 years of age that were hospitalized with influenza A infections had febrile seizures (Chiu et al., 2001). Pregnant women are at slightly increased risk of serious pneumonia (Laibl & Sheffield, 2005).

Developments that have reduced the risk of influenza

In the recent decades, a number of developments have led to a reduction in the number of influenza cases and deaths due to influenza. The physical resistance of populations has increased by improved general health status. In addition, a decrease in the number of persons per household has probably resulted in a reduced transmission rate and spread of the influenza virus. Finally, the seasonal flu epidemics have been fought by increased vaccination in particular risk groups.

Increased mobility and social contacts increase the risk

In contrast to developments that have reduced the risk of influenza, there are developments that have increased this risk. One of these developments is the rise of social contacts, for example by disco visits, participation in major (sports) events and the accommodation of young children in nurseries. In addition, increased (global) mobility may have led to a greater or faster spread of the influenza virus.