An influenza pandemic starts with the circulation of an influenza virus that is totally different from the others already in circulation, that starts to infect humans and efficiently spreads from person to person.
Since the virus is so different, fewer people than normal will be immune to the virus, which will spread rapidly and infect an unusually high number of people. The word pandemic means that the virus is spread over the entire world, or large parts of it.
During the period from 1510 up to the present, we are aware of 18 influenza pandemics. Four of the pandemics have occurred in the last 100 years. These are commonly referred to as:
- Spanish flu (1918)
- Asian flu (1957)
- Hong Kong flu (1968)
- Swine flu (2009)
Where do these new viruses come from?
Humans are not the only creatures that get influenza. A wide variety of influenza viruses circulate among birds, as well as some other mammals. Occasionally, an influenza virus will infect between species. Such viruses are usually poorly adapted to the new host, and further spread of the infection will be less effective.
A virus can adapt through major or minor changes in its own genetic make-up (i.e. mutations), or it can take a shortcut by swapping one or more genes with influenza viruses already adapted to the new host. The pandemic viruses from 1957, 1968 and 2009 were all such strains, also called reassortants.
In order for such a mixing of two different viruses to occur, they must exist side by side in an individual; in fact, inside the same single cell in the individual. Pigs are such a species where this can easily occur, as they are fairly receptive to influenza viruses from both birds and humans.
If such a virus is suited to infect and cause disease among human beings, and on the outside has molecules that are completely unlike those that are on the outside of the influenza viruses already circulating among humans, the virus will thrive, possibly causing a new pandemic.
Why is there concern about new influenza viruses?
A new virus that spreads easily will lead to a great many people becoming ill. If no one has had the disease before, then no one is immune against the virus.
There is a risk that a new virus could lead to serious disease, causing many deaths. We don’t know how dangerous viruses are in advance. Viruses can also change characteristics, and become even more dangerous when they have circulated among humans over time.
Society can face considerable challenges during an influenza epidemic. If many people require care at the same time, the health service may have a hard time providing adequate care for patients. This challenge becomes even greater if many health workers become ill. There are also many other important functions in society that will suffer if many people are away from work at the same time.