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Everyone seems on edge about the latest outbreak of avian flu. People in China are told to kill their poultry in order to keep the virus from spreading. Meanwhile people in the United States watch on in fear that the avian flu will come to their shores. So what exactly is all the hubbub about? After all, every winter millions of people come down with bird flu.

Origin of Influenza

Influenza, the term, came into use around 1504, though it had little to do with the virus. It came from the word: influence. At the time, influence meant: 'the ethereal power of the stars acting on men.' It was basically a supernatural way of explaining the effects of disease on people at the time. During a particularly nasty outbreak of the flu in Europe during 1743, the term was officially attached to the name of the disease. 96 years later, the term was shortened to just: flu.

The flu, itself, is a whole family of viruses called Orthomyxovirids. They are a diverse family that are commonly found in the guts of birds. The specific type of viruses that infect birds, are called type A flu. It was one of these type A's that was believed to have infected people a long time ago. Thus giving us, the flu for the first time. Though the virus that initially infected man, has long since evolved into a variety of human specific strains, the initially origin appears to lay squarely among birds. As such, all human flu bugs could, technically, be called: avian flu.

Pathogens and specificity

Pathogens are viewed as being any living organism that is capable of causing a disease. It is a term that is generally reserved for bacteria, fungi and viruses. Pathogens are usually very specific in who, or what, they infect. This has a lot to do with the way in which they are constructed.

Both bacterial and viral outer structure, consist of a receptor binding proteins. These proteins give the pathogen a certain geometry. This geometry allows the pathogen to attach to complementary receptor sites on the cells of the critter that they are trying to infect. Because of the wide variety of life forms on the planet, each cell type has a different arrangement of receptors. Most of the time, the pathogen's geometry will not fit these receptors, and the critter remains immune. Only those unlucky few species, whose cell receptors do fit, are the ones that have to suffer the infection.

Occasionally, though, a new pathogen comes along that has a geometry that is general enough to allow it to latch onto many different species. These are the pathogens that are often the more deadly.

Influenza is one of these general viruses. It is capable of infecting most bird species. It's also very good at doing what all life forms do. It evolves. This has allowed it to cross multiple species barriers, and jump from birds, to people, to pigs, cows, and horses. Thus making influenza a very cosmopolitan virus family. This still doesn't explain all the worry about this recent outbreak of avian flu though. For that, one must go back in time to 1918, and the Spanish flu.

It was the close of World War I, and the world appeared to be returning back to a more peaceful state. Then, in various parts of the globe, people started coming down with a particularly virulent form of the flu. This was a unique case though. Instead of the very young, and elderly dying, it was affecting young men and women instead. Usually these are the most immune to the effects of the flu. By the end of 1918, this form of the flu had killed ~50 million people. It was the largest pandemic (worldwide epidemic) in recorded history. So what happened?

Normally when one gets the flu, it is more of a hassle than anything else. This has a lot to do with the fact that the flu types we normally catch, are viruses that have infected us before. They have changed just enough so that they can infect us again, but they still remain recognizable to our immune system. As such, our bodies can keep the virus in check, and then eventually eliminate it. The 1918 flu, though, was different. It is now largely believed to have been a case where a new flu virus had hopped species. It went from birds to humans, possibly after circulating and hybridizing inside pigs (which can catch both bird and human versions of the flu). This new bug was completely alien to our immune systems and thus, took many completely by surprise.

This is what has many scared about this newest avian flu virus (dubbed: H5N1, for the specific proteins found on it). It has proven to be particularly virulent among birds, and the few cases of it infecting people have many worried that another pandemic is on the rise.

Zoonoses.

Influenza is a type of disease referred to a zoonosis. It means that it can be transmitted from one animal group, to another. Zoonotic diseases used to be further broken up into those that humans catch from other animals (anthropozoonoses) and ones that other animals catch from humans (zooanthroponoses). Unfortunately, both terms have been misused and confused so much, that neither is particularly favored anymore. Now they are all viewed as zoonotic diseases. In the end this makes the most sense, as human beings are animals anyway. To break things up any further, just seems excessive.

The flu is not the only zoonotic disease that humans get from other animals. Our primate cousins have given us quite a few different diseases including: malaria, hepatitis B, Dengue fever and lymphoma. Of course the most infamous of these zoonotic diseases would probably be HIV.

Though there are those that would like to believe that the Human Immunodeficiency Virus was a genetically engineered weapon that was released among the African populace (they give far too much credit to genetic engineers, who are proud enough to make yeast that can fluoresce), the simian origin of HIV is pretty well established. HIV has close ties to the simian version: SIV (Simian Immunodeficiency Virus). The big difference between the two, besides their first letters, is that SIV rarely kills the apes it infects. In fact, many apes are capable of carrying viral loads equivalent to those seen in humans with advanced AIDS, yet rarely show any signs of trouble. This suggests that the host and the pathogen have been doing this for a very long time, and the host's body has found a way to handle the virus. Humans only recently acquired HIV. As such, our bodies have yet to 'learn' how to deal with the threat that this virus poses. Which is one reason why HIV is so very virulent at the moment.

These are just some of the diseases that other animals have given to humans. But what of the reverse? What have we given our animal brethren?

Many of the 'classic' diseases that most humans catch, are ones that we are capable of giving to our primate cousins. This includes the flu, measles, chicken pox and tuberculosis.

One particularly nasty disease that we are capable of transmitting is the infamous Foot and Mouth Disease (FMD). FMD rarely affects humans, but it does use us as a carrier for it. The disease can hang out in our nasal passages, throat, and on our clothing. It usually infects various forms of livestock (cattle, pigs, sheep, goats). Symptoms usually include fever and prominent sores on the feet and mouth (hence the name). Most infected animals do survive. Only ~5% die from the disease.

The second disease is far more pernicious, and the victims have us to blame. It is the coral disease referred to as: white pox. This disease can kill up to 10 square centimeters of coral a day (~120ft a year). Over the past decade over 90% of Caribbean reef coral (Acropora palmata), have died. The culprit behind it is the little human gut bacterium: Serratia marcescens. While humans can occasionally fall victim to this bacterium, it usually doesn't infect us. Instead it lives in our guts and gets expelled in our feces. Improper sewage treatment has resulted in human excrement flowing out into the Caribbean, where the newly released bacterium has infected the local coral.

So remember; the next time you start to feel under the weather, don't worry about coughing on your dog. Chances are, your canine pal probably won't get it. Unless, of course, it is the flu.