Bacteria are single-celled organisms, capable of reproducing on their own. Different kinds of bacteria thrive in a variety of different environments (our bodies being just one of many examples), but they are all "alive" by anyone's definition of the word. Because they are alive, they can be killed. Therefore, in addition to your body's own immune responses to get rid of bacteria, you can kill them with antibiotics. Viruses are not really living organisms (my high school biology classes stressed that there was a debate about whether we could actually say they were alive). They are bits of DNA or RNA inside a protein shell (much smaller than bacteria), but they cannot reproduce by themselves. That's why they infect your cells: to take over the structures that your cells use to reproduce and use them to make more viruses. Since they're not technically alive, they can't be killed. So antibiotics do not work on viruses and taking them is a waste of time. You just have to let your immune system track them down and get rid of them. "Get rid of" and "kill" are my own cool scientific terms... just remember that they refer to two separate processes! My degree is in French and Spanish literature, not in biology!

Bacteria and viruses vary greatly in size- bacteria can be anywhere from 10 to 100 times larger than viruses. Basically, bacteria contain everything for them to reproduce- genetic information in the form of DNA or RNA, ribosomes (necessary for translating the genetic information into enzymes required for its survival), and plasmids which convey extra genetic information for a variety of purposes (bacterial "sex" for example- bacteria normally reproduce asexually, but with certain plasmids genetic material can be passed between bacteria). Viruses, on the other hand, generally only have genetic information, have no way of replicating that information and also lack the machinery needed for making its own proteins. They need to infiltrate living cells to be able to reproduce. You may have seen pictures of viruses looking like moon-landers- the legs attach onto the surface of the cell, then the genetic material contained inside the head of the virus is injected into the cell. This genetic material can either use the cell's machinery to produce its own proteins and/or virus bits, or it can be integrated into the cell's DNA/RNA and then translated later. When enough "baby" viruses are produced the cell bursts, releasing the new viral particles. They also differ in structure- bacteria have cell walls (in addition to their cell membranes), which contain peptidoglycan polymers and provide structural definition. Another answer mentioned the fact that antibiotics can kill bacteria but not viruses. This is because many antibiotics interfere with the production of peptidoglycans for cell walls. Viruses contain a simple protein coat on the outside. They lack peptidoglycans, therefore are not affected by antibiotics.

1) "Viruses are tiny geometric structures that can only reproduce inside a living cell. They range in size from 20 to 250 nanometers (one nanometer is one billionth of a meter). Outside of a living cell, a virus is dormant, but once inside, it takes over the resources of the host cell and begins the production of more virus particles. Viruses are more similar to mechanized bits of information, or robots, than to animal life. Bacteria are one-celled living organisms. The average bacterium is 1,000 nanometers long. (If a bacterium were my size, a typical virus particle would look like a tiny mouse-robot. If an average virus were my size, a bacterium would be the size of a dinosaur over ten stories tall. Bacteria and viruses are not peers!) All bacteria are surrounded by a cell wall. They can reproduce independently, and inhabit virtually every environment on earth, including soil, water, hot springs, ice packs, and the bodies of plants and animals. Most bacteria are harmless to humans. In fact, many are quite beneficial. The bacteria in the environment are essential for the breakdown of organic waste and the recycling of elements in the biosphere. Bacteria that normally live in humans can prevent infections and produce substances we need, such as vitamin K. Bacteria in the stomachs of cows and sheep are what enable them to digest grass. Bacteria are also essential to the production of yogurt, cheese, and pickles. Some bacteria cause infections in humans. In fact, they are a devastating cause of human disease." Source and further information: http://www.drgreene.com/21_527.html 2) "There are a number of differences between a virus and bacteria, or more appropriately, viruses and bacteria, including their relative sizes, reproduction methods and resistance to medical intervention. Bacterium, the singular form of the word bacteria, is a one-celled living organism, with complete sets of both ribonucleic acids (RNA) and deoxyribonucleic acid (DNA) genetic codes. A virus is little more than a section of RNA or DNA covered by a protein shell. Bacterium, therefore, is at least a hundred times larger than a typical virus." Source and further information: http://www.helium.com/items/936060-the-difference-between-bacteria-and-viruses Further information: - "How to Know the Difference Between Bacteria and Viruses": http://www.wikihow.com/Know-the-Difference-Between-Bacteria-and-Viruses - "Bacterial vs. Viral/Respiratory Tract Infections": http://www.cchs.net/health/health-info/docs/3500/3500.asp?index=11751&dpath=http:/www.cchs.net/health/health-info/docs/3500/3500.asp?index=11751 - "What is the difference between bacteria and viruses? ": http://www.extension.org/faq/26425