The influenza viruses are a group of nasty viruses which have RNA (ribonucleic acid), instead of DNA (deoxyribonucleic acid) as their basic genetic material. Here is where the problem lies: DNA is good stuff. It is usually very stable; and, when it is replicated (duplicates made by copying each of the two strands to generate two sets of two strands each), the thing that does the copying (an enzyme called DNA-dependent DNA polymerase - means the enzyme must recognize DNA to make a replicate of the DNA), has the ability to do a "spell-check" on the entire reproduced "document." This "spell-checking" ability, makes certain that any errors which may have occurred in replicating the DNA, are corrected. Therefore, any time a cell in our body divides, we can be confident that the exact same "document" of DNA is replicated, and each daughter cell receives an exact duplicate of the original DNA. The same thing is true for any organism whose basic genetic material is DNA.
Unfortunately, many organisms have RNA as their basic genetic material; and, as stated above, influenza viruses are an example. The common cold-causing viruses, are also RNA viruses (poliovirus belongs to the same virus family, the picornavirus family). HIV is another example (however, is a retrovirus - makes DNA from its RNA genome). So, what's the deal here? Why does RNA in viruses make any difference to us?
There is no "spell checking" of the RNA document when viral genetic material is replicated. The enzyme which copies RNA (RNA-dependent RNA polymerase - means the enzyme must recognize RNA to make a replicate of RNA) does not have this ability. Consequently, errors can be generated and the copied material is said not to have proper fidelity (poliovirus may be an exception - the RNA genome of this virus is replicated with very high fidelity - and it is said that poliovirus is very antigenically stable; therefore, there are only three poliovirus "types" identified - my thanks to Tom Terry, University of Connecticut for the reminder). The fidelity of poliovirus RNA replication is the reason that immunization against this virus has been so successful - see Good News about a Bad Bug, this series).
In the worst case (for the RNA organism), the errors may be lethal, and the organism won't survive. In the best case (again for the RNA organism), the errors won't lead to anything much, except that the organism may behave a little differently, or, may "look" a little (or a lot) differently. It is the behavior and "look" differences that are a problem for us.
When a particular influenza virus replicates the RNA genetic material many, many, times (allows many individual influenza viruses to be made), errors appear. The indiviual virus among the group, which has the errors, now may "look" very different from the rest of the group. This virus, however, is still an influenza virus, with all of the properties of such a virus; however, now there exists a new strain of influenza virus.
If we become infected with this new viral strain, our bodies have never, ever, seen this particular "looking" virus before. Consequently, our immune system must start fresh to respond against this infection. An analogy would be that our immune system has had no previous "practice" in responding against this virus. The response is therefore relatively slow - may take many days to mount an effective response - thus, by this time, we may be very, very, ill. Consequently, for the already ill, elderly, and very young, the influenza virus can be deadly.
Because of this "changling" capacity of influenza viruses, there is a world-wide network which monitors all of the influenza cases which appear in the world! Anytime there appears to be an influenza epidemic beginning somewhere, the culprit virus is identified, isolated, purified, and vaccine prepared against it. And, stocks of the virus are kept for future reference, to allow quicker production of vaccine, the next time this "changling" appears. There are records kept of the rate-of-change that influenza viruses undergo, to try and help predict when, and which kind of, an influenza virus "changling" may appear. You may wish to examine a site which has all kinds of information about the different viruses, so, take a look at: Complete Virology Information, Tulane. And, you may wish to examine The World Health Organization's (WHO) Page on Disease Outbreaks around the world - including WHO influenza surveillance activities.
I expect that you are aware of the recent concern about a new strain of Influenza virus Type A - H5N1 - which emerged in Hong Kong, China. The acronym, H5N1, stands for: Hemagglutinin type 5 and Neuraminidase type 1 (genetic types associated with proteins within the outer coat of the virus which are required for the virus to be able to infect a cell).
This strain of virus is apparently directly transmissable from birds (chicken) to humans. In certain cases, a human strain of Influenza virus Type A can infect a pig... and the pig may also simultaneously be infected by a pig strain (swine). Under this circumstance, the two strains of virus can swap some genetic information - which can lead to a new, emerging strain which is capable of infecting humans,e.g., a particular strain of "swine flu" . Similarly if a bird (avian) strain of Influenza virus Type A infects a pig and if the pig is also infected with a human strain of Influenza virus Type A, under this circumstance, genetic material can be exchanged between these two virus strains.. the human strain can acquire new characteristics (from the bird virus) and present a new, emerging strain which humans have not before encountered.
The issue with the recent occurrence in Hong Kong, is that there appears to be a directly-transmissable Influenza virus Type A from birds to humans, without the usual requirement that a pig or some other domestic animal be an intermediate to allow for genetic material exchange. Please see: The World Health Organization's comments on H5N1 and other Influenza strains, and Some Additional Influenza Facts. In addition, the following site offers an excellent review of the virus family, Orthomyxoviridae to which Influenza virus Types A/B/C belong. Please see: Myxoviruses by Dr. Alan Cann, Department of Virology and Immunology, University of Leicester, United Kingdom.
Always check with your physician/health-care provider, to determine if
you, or those important to you, may be at particular risk if influenza is
contracted. The deaths which occurred during and after the years of World
War I (20-million world-wide), that were caused by influenza, far
exceeded the loss of life due to the war itself. Therefore, talk to your
physician/health-care provider for advice about whether or not you are a
candidate for immunization. Also, if the "flu" is
contracted, be sure to avoid administering fever-reducing medication which
contains salycilates (Aspirin) to children. There is the possibility of
acquisition of a neurologic condition known as Reye's syndrome if children
receive salycilates when infected with Influenza virus Type A