What is the respiratory tract?
The respiratory tract is made up of the entire system that allows you to breathe. From your nose and throat, to the sinuses, down the trachea to your lungs, the respiratory tract carries the air that supplies the oxygen necessary for life. All of the respiratory tract is lined with the same type of moist membrane and functions as a complete system. Because the respiratory tract functions by inhaling air from the outside world, infections of this system are very common.
What types of germs cause respiratory tract infections?
Two major types of germs, or microorganisms, cause infections in the respiratory tract.
Most respiratory tract infections are caused by viruses. Viruses are so small that they cannot be seen, except with a very sophisticated instrument called an electron microscope. And, most important, viruses are not affected at all by common antibiotics.
Bacteria are very different from viruses. Bacteria are many times bigger than viruses, big enough that they can be seen with the type of microscope found in any high school laboratory. Finally, bacteria can be treated by antibiotics. In fact, antibiotics are sometimes called antibacterials for that reason.
Understanding the differences between these is crucial to understanding why some infections are treated with antibiotics and others are not.
Common types of respiratory tract infections
The common cold
Colds are the best example of a viral illness. There are several different families of viruses that can cause the common cold - the most common family is called rhinovirus ("rhino" means nose). Over 150 rhinoviruses cause infections in humans, so even though you develop immunity to each particular virus after you have had it, that still means you can have dozens and dozens of colds over your lifetime with lots of viruses left over.
The symptoms of a common cold are well known: runny nose, scratchy throat, aching muscles, a bit of a cough, any or all of these, lasting over a week or so. During the course of a cold, your nasal discharge tends to change from watery to thick and green - this does not mean that a different infection has developed, it is just part of the cold.
Because the common cold is caused by a virus, antibiotics are useless. No treatment makes any difference in the duration of the infection, so the only reason for taking any kind of therapy is to reduce the symptoms. Many people find it useful to suck on throat lozenges, take over-the-counter painkillers for the aches and pains, drink lots of fluids, and get lots of rest.
Influenza (‘flu’)
Influenza, while also caused by a virus, is very different from the common cold. Influenza lasts longer (usually two weeks), has different symptoms (severe headaches and aching muscles, persistent dry cough and overwhelming fatigue) and is much more severe. In people who are frail because of age or other medical conditions, influenza can be fatal.
Acute sinusitis
Infections of the sinuses often follow, or arrive as a complication of the common cold. Typically, after you have had the cold for three or four days, you find yourself getting worse rather than better, with pain below your eyes, toothache, and fever. Tapping over the cheekbones or bending over to tie your shoelaces will often cause severe pain.
Acute sinusitis is usually a bacterial infection, so antibiotics are very useful. This is often a stubborn infection, so antibiotics may need to be given for 10 or 14 days. As well, many doctors prescribe a medication to open the nasal airway and reduce congestion.
Bronchitis
Bronchitis, again, is often a complication of the common cold. Like many respiratory tract infections, bronchitis is more likely to develop in smokers, because smoking impairs the ability of the respiratory tract's lining to clear out germs. Bronchitis shows up as a persistent cough, usually with thick, coloured phlegm, but without a fever, following a common cold.
Medical experts feel that many cases of bronchitis are caused by viruses and will get better without antibiotics. In some cases an antibiotic might be prescribed, especially if you are a smoker, elderly, have other health problems, or continue to get worse as days go by. If you are a smoker, quitting smoking now will help you get better faster and reduce the chance of bronchitis in the future.
Pneumonia
Pneumonia, the most severe of the common respiratory tract infections, involves an infection in the lung itself. If you have pneumonia, you are usually quite ill with high fever, cough, shortness of breath, chest pain, and weakness.
Pneumonia is usually a bacterial infection, so it should be treated with antibiotics. If you are generally healthy, and younger, then treatment at home is usually suggested. In contrast, hospital treatment is usually required if you are elderly, have other medical problems, or are more seriously ill. 
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