I've been asked by a reader to run a column I did a while ago on the differences between bacterial and viral infections. This is apropos this time of year as we head into cold and flu season.

When we were kids, we learned the things that made us sick were just "germs." Those who listened in science class discovered there is much more to the microscopic world around, on and in us. Bacteria and viruses are the two groups of microorganisms that cause most infectious diseases in humans.

Bacteria are single-celled organisms that live virtually everywhere on Earth. There are actually more bacteria in and on our bodies than there are human cells. They can divide and multiply on their own using their cellular machinery. Bacterial biochemistry is different than that of human cells. It is these differences that allow scientists to develop medications to slow them down or kill them.

Antibiotics kill bacteria directly or by altering their ability to multiply. Since bacteria are different from human cells, the antibiotics generally don't harm our bodies directly. However, antibiotics are non-selective and also kill bacteria that are beneficial to our health. In fact, about 99 percent of the bacteria in our bodies are good guys aiding various body functions and helping to make vitamins.

Most bacteria live in our digestive tract and are key for digestive health. The incidental killing of these bacteria can lead to problems like severe diarrhea. Killing off beneficial bacteria can also allow other harmful organisms like yeast and even other bacteria to move into the space left by those that were killed. Some of these infections can be extremely serious and even fatal.

Strep throat, ear infections, lung infections (bronchitis & pneumonia), bladder infections, sinus infections, skin infections and intestinal infections are examples of some illnesses caused by bacteria. Even though these infections are bacterial, a healthy immune system is designed to clear the infections in most cases without the need for antibiotics. In fact, the overuse of antibiotics has led to bacteria that can no longer be killed by the drugs - an extremely serious problem, and one of the reason doctors are appropriately cutting back on prescribing antibiotics.

Viruses, on the other hand, are not able to reproduce on their own. They are made up of genetic material (DNA or RNA) contained in a protein coat or "capsid." Unlike bacteria, they lack the machinery to replicate. They require a host, such as a nice warm human body, to reproduce. They do this by infecting our cells and using the cells' biochemical machinery to make new viruses.

In contrast to bacteria that can usually be killed with antibiotics, viruses are much more difficult to eradicate. Since these invaders live and reproduce in our cells, it's difficult to direct a drug to kill them without harming our own cells. There are a few drugs known as anti-virals that can target the reproductive cycle of viruses.

Viruses cause illnesses such as the common cold, respiratory influenza, chickenpox and gastroenteritis or "stomach flu." Something that most people don't know is that viruses, not bacteria, cause most cases of bronchitis, sinusitis and even pneumonia.

Since viruses are the predominant cause of these illnesses, they will usually improve on their own. This is why doctors often say, "if I treat your cold with an antibiotic, you should get better in a week. If you just treat your symptoms and let your immune system do its job, you should get better in seven days." Many people have the false notion that antibiotics cured their viral infection, when in fact the infection would have gone away just as soon without them.

So the next time you get sick with a respiratory illness, don't immediately jump to the conclusion you need an antibiotic. Treat your symptoms and let your immune system do the job it evolved to do.

Dr. John Roberts is a family physician. He is also one of the owners of The Paper of Montgomery County. Send him your question today by e-mail at thedoctor@thepaper24-7.com.