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Wikipedia has the following statement on its Upper respiratory tract infection page:

In United States URIs are the most common infectious illness in the general population.

What is it about the upper respiratory tract that makes it so vulnerable to infections?

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  • $\begingroup$ I'd argue it's the exposure to foreign bodies which predisposes the upper respiratory tract to infection. Though, it's not until these invaders begin penetrating the tissue or in some way evading the immune response that we see any sign of infection. All things equal, you could even argue the entirety of the alimentary canal is at risk. However, in most instances, all is not equal, and most foreign bodies are in for some real punishment trying to invade those areas. $\endgroup$ – CKM Feb 18 '15 at 23:43
  • $\begingroup$ That makes sense, but beyond upper respiratory tract and the digestive system, there are many other tissues in constant contact with foreign bodies, like most of our skin, eyes, lips, the ear canal; and even though there is more air flow in our respiratory system, which could help explain its vulnerability, we don't see the same rate of infection along the lower respiratory tract. That's why it seems probable to me that there's something about those tissues that makes them so reliably exploitable by pathogens in well known cyclical patterns. $\endgroup$ – Smig Feb 19 '15 at 17:37
  • $\begingroup$ Well, the lower respiratory areas have whats known as a "ciliary escalator," as well as IgA in the mucus and alveolar macrophages. Which, you find that lower respiratory infections are more relatively severe given the evasion of such a complex of defenses. Other parts of the body, microbes are at competition with your normal flora, immunoglobulins in secretions like saliva and tears, lymphoid tissue, etc. $\endgroup$ – CKM Feb 19 '15 at 21:15
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Mostly because it is in almost constant contact with aerosols from the outside world (with regard, of course, to a thin layer of mucus), therefore any pathogens they might contain.

You also have to remember what separates an airborne pathogen from any other pathogens- infection location. A pathogen is airborne when it infects and irritates the throat or nasal passage enough to make the organism's normal deterrence functions kick in. In most cases this would be coughing or sneezing, which produce contaminated aerosols for other organisms to inhale through their own upper respiratory tract.

I hope this answers your question!

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    $\begingroup$ It partly answers the question, but only partly. The external skin is also in almost constant contact with aerosols (and other forms of contamination) from the outside world, but it is not nearly as vulnerable to infectious agents as the upper respiratory tract. A full answer to the question would explain what constraints prevent the upper respiratory tract from being as well defended as the external skin. $\endgroup$ – Corvus Feb 26 '15 at 3:46
  • $\begingroup$ The external skin is most definitely not in contact with aerosols (the living part at least). The first layer of skin we have is the epidermis, the first layer of the epidermis is the stratum corneum, which is composed of dead skin cells that form an infection-proof layer over the living skin cells (which is why cuts get infected). I apologize for the confusion, I was under the impression that this was common knowledge. $\endgroup$ – CDB Feb 26 '15 at 4:04
  • $\begingroup$ Sure -- but this just begs the question of why a similar infection-proof layer could not coat the upper respiratory tract as well. $\endgroup$ – Corvus Feb 26 '15 at 6:39
  • $\begingroup$ Well, think about it. If there were a layer of dead cells in our lungs, oxygen would not be able to enter the alveoli and diffuse into the blood. However, you must remember that there actually is a layer like this, the mucus layer of course (oxygen can diffuse even while the mucus is present), but it does not protect nearly as well as a layer of skin, as we can see by the infection rates. $\endgroup$ – CDB Feb 26 '15 at 16:47
  • $\begingroup$ This doesn't answer the question either, because the question was asking about the UPPER respiratory tract, not the lower. The upper respiratory plays little if any role in oxygen exchange. $\endgroup$ – Corvus Feb 26 '15 at 17:03

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