What is the difference between viruses and bacteria?

Dr. Coleen Cunningham is chief of pediatric infectious diseases at Duke University Medical Center. She explains that even though at first glance all of those stuffy, runny, sniffly coughs may look the same, the two main types of bugs that make a person sick are actually completely different. Questions and answers have been edited.

Q: What are viruses? Viruses are very small infectious agents that can only replicate inside other living cells.

Q: What are bacteria? Bacteria are slightly larger than viruses and are made up of a single cell. They are very simple organisms, and most can replicate outside other cells. They are virtually everywhere.

Q: What are the key differences between the two? One difference is size; viruses are smaller. Another difference is how they work. A virus only works by invading a cell within an organism and taking over that cell's machinery to reproduce itself. By itself, it doesn't contain all ability to do everything it needs to survive and replicate. It essentially has to parasitize that other cell.

Q: Which are most likely to make you sick? The answer to this question would depend on what you were exposed to. Most of the time when we have a mild illness, we have a viral infection. Most sore throats, all "colds," bronchitis, flu and upset stomachs are due to viruses. But some pneumonias, ear infections, skin infections and bladder infections are caused by bacteria. Meningitis, pneumonia, diarrhea and others can be caused by either. Certainly, there are bacteria that can kill you, but there are also viruses that can kill as well.

Q: How do you treat viral illnesses versus bacterial infections? Again, it depends on the virus or bacteria. You treat bacterial infections with an antibiotic. You treat viral infections with an antiviral. However, some infections don't need treatment at all, and for many viruses, in particular, we don't have very effective therapy.

Q: What are the dangers of misuse and overuse of antibiotics? There are three primary dangers of antibiotic misuse or overuse. No. 1 is cost: Drugs are expensive, and anything spent on unnecessary meds is at best, a waste of money. No. 2 is side effects: All drugs have toxic side effects. If it is unnecessary, then it isn't worth taking a chance of an adverse effect. No. 3 is the selection of bacteria that are resistant to antibiotics. Once drugs are widely used, the bacteria develop resistance to those antibiotics. Eventually, a drug may become not useful. We have this now with some staphylococcus and enterococcal organisms (among others) where there are now very few drugs that can be used.