It's that time of year when respiratory illnesses are running amok. As usual, patients are bombarding their doctors with requests for treatment for these illnesses. Frequently, the requests are for antibiotics. Patients may think their doctor is not doing his or her job when the patient's request is denied. In fact, the physician is very likely practicing good medicine.

First of all, I want to briefly review what antibiotics do. When I refer to antibiotics, I am generally referring to medications that help kill bacteria. Bacteria cause many respiratory infections. Viruses, however, cause the vast majority of infections such as the common cold, sinusitis, bronchitis and sore throats. Antibiotics do NOT kill viruses.

Therefore, most people who present with respiratory illnesses do not need antibiotics to get better - they will make no difference in the course of the illness. Their immune systems generally will do the job and clear the viruses from the body. There is a saying in medicine that, "a patient should see improvement in symptoms from a virus in a week with antibiotics and in seven days without them."

The over-prescribing of antibiotics for illnesses not caused by bacteria can lead to a very serious problem - antibiotic resistance. Most people are familiar with this concept, but don't know the details of what it means to public health.

The discovery of penicillin was a huge advance in medicine. People who would have died or developed serious complications from simple things like strep throat no longer had to be as fearful for their health.

However, as the use of penicillin became more commonplace, physicians began to notice that it was not as effective in treating infections - the bacteria were staying one step ahead of medical science. The bacteria were developing resistance to penicillin.

Resistance develops through natural selection. A person may have millions of a particular bacterium inhabiting their respiratory tract (or any other area of the body). Somewhere in those millions of bacteria, there may be a very few that have developed a genetic mutation giving them the ability to ward off the effect of a particular antibiotic.

When a patient who is infected with these bacteria takes the antibiotic, it will kill off only the bacteria that are sensitive to it. Those few who had the mutation will be spared to reproduce and spread. Typically, killing off most of the bacteria will allow the patient to recover. The problem occurs the next time these remaining bugs cause a problem - they are now too numerous to be killed by the original antibiotic.

Resistance can also develop if a patient does not complete a course of antibiotics for an infection. This happens often when someone starts to feel better and quits taking the medication. The bacteria that are most sensitive to the antibiotic are killed off early in the course of treatment. When the patient stops taking the medication the ones that have more resistance survive to reproduce and refine their ability to resist the antibiotic.

Resistance would not be a major problem if the bacteria did their business, killed the patient and that was that (as long as you weren't the patient). However, bacteria are more than happy to spread to a new host and set up shop - hence the public health threat of these resistant bacteria spreading throughout the population.

To make matters even worse, some bacteria have the ability to share their antibiotic resistance with other bacteria. They do this by sharing with each other the pieces of their DNA that code for resistance.

From day one in the science of infectious diseases, it has been a constant battle of man against nature. As bacteria become resistant to one antibiotic, another must be discovered that will kill the resistant bugs. Scientists have been staying ahead of nature for a long time, but new antibiotics are becoming harder to find and develop - the bacteria are starting to win the war.