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Influenza is a highly infectious viral disease which can occur as a pandemic, epidemic, outbreak and in form of sporadic cases. A majority of human infections are caused by either type A or B influenza viruses. Type A has been associated with widespread epidemics and pandemics, while type B has been infrequently implicated in regional epidemics. Influenza type C infections cause only a mild respiratory illness.
Yearly influenza epidemics can affect all populations, but children younger than the age of two, adults older than 65, as well as the people with chronic medical conditions or weakened immune systems bear the highest risk of complications. Annual attack rate is estimated at 5–10% in adults and 20–30% in children.
In temperate climates seasonal epidemics of the virus occur mainly during the winter time, compared to tropical regions where it may occur throughout the year, resulting in much more irregular outbreaks. The epidemics caused by the influenza virus are estimated to result in about 3 to 5 million cases of severe illness, and up to 500 thousand deaths worldwide.
Influenza virus is transmitted among humans in three main ways: by direct contact with infected individuals, through contaminated objects (such as hairbrushes or towels – often called fomites) and by inhaling virus-laden aerosols. The contribution of each mode of transmission to overall spread of influenza is not known.
The production of aerosols that contain virus particles is necessary for the respiratory transmission. Speaking, singing and even normal breathing can produce an adequate amount of aerosols, while sneezing and coughing lead to more forceful expulsion.
Aerosolized particles produced by the aforementioned activities have different sizes. The largest droplets fall to the ground within a few meters and will infect only those in the immediate vicinity. Distance that other droplets cross is often determined by their size.
The droplets that are between 1 and 4 microns in diameter are known as “droplet nuclei”. These remain suspended in the air for long periods of time and not only they have the ability to travel long distances, but they can also reach the lower parts of the respiratory tract. Inhalation of droplets and droplet nuclei situates influenza virus in the upper respiratory tract, where it has the opportunity to initiate infection.
Nasal secretions that contain virus particles are responsible for transmission by direct contact or via contaminated objects. An infected individual most often touches the nose or conjunctiva, thus placing virus on the hand. Any ensuing contact (for example, shaking hands) can transfer the virus to another person, who will then infect themselves by simply touching their eyes or nose.
In addition, the virus transmission also happens upon touching other objects with contaminated hands. Researchers have shown that up to 60% of objects from homes and day care facilities were shown to harbor influenza viral RNA. Infectious influenza virus may also persist on paper bills for several weeks, which is another possible way of spread.
Viral glycoproteins hemagglutinin (HA) and neuraminidase (NA) change periodically due to sequential evolution within immune or partially immune populations. Such periodic changes result in antigenic mutants of the virus, which are selected as the predominant virus because the antecedent virus is suppressed by specific antibody arising in the previously infected population. Such cycle then repeats continuously.
One type of change is called antigenic drift, which occurs when small changes in the virus happen continuously over time. Surface antigens of the virus undergo minor change, and since the protection that remains from past exposures to similar viruses is incomplete, an epidemic can ensue. Antigenic drift can occur in all three types of influenza virus.
On the other hand, antigenic shift represents an abrupt, major change in one or both surface antigens (HA or NA) that occurs at varying intervals. It is a result of a genetic recombination (an exchange of gene segments) between influenza A viruses – most often those that affect humans and birds. If the virus spreads efficiently from person to person, an antigenic shift may result in a worldwide pandemic.