In essence, antibiotics are selective poisons used to kill bacterial cells. The term loosely translates to "against life." In some senses, all things that kill cells are antibiotic; this includes poisons and toxins. Chemotherapy is antibiotic because it kills cancer cells, and unfortunately some human cells with it. Yet in most definitions, antibiotics are medications people take when they have bacterial infections.
Antibiotics fight infection by either killing bacteria outright or by inhibiting the growth or development of bacterial cells. The former type is called bactericidal, and the latter, bacteriostatic. The goal of the antibiotic, which can be made of naturally occurring fungi or chemical compounds, is to harm bacterial cells that are making people ill, without harming human cells.
Scientists classify antibiotics by either how they work or the bacteria against which they are most effective. When bacteria can be killed, the antibiotic disrupting the membrane of bacterial cells normally does this. They can also inhibit bacterial growth through keeping bacterial cells from making proteins and acids that they need for survival and reproduction.
In order for antibiotics to be effective, it's important to understand the structure of bacterial cells, which fortunately differs significantly from most animal and plant cells. The proteins or enzymes and the DNA structure of bacterial cells are targeted through either natural or chemically produced antibiotics, and at the same time, researchers look for compounds that will select only bacteria for attack and not plant and animal cells. It should be noted that these medications only fight infections caused by bacteria. They are completely ineffective against infections caused by viruses or fungi.
Some antibiotics are called broad-spectrum because they can be used to kill or impair many different types of bacteria. Others are narrow-spectrum and only have uses against a few specific types of bacteria. Most common infections, like strep or staph are relatively easy to treat with broad-spectrum medications. Other, more resistant bacteria may require a narrow-spectrum treatment.
Unfortunately, humans host many types of bacteria, and some types are quite beneficial. Broad-spectrum antibiotics leave human cells alone, but often have an effect on the good bacteria people carry. This can result in complications from taking the medication, like getting yeast infections or having diarrhea.
Trends in medicine include verifying that people have a bacterial infection prior to prescribing antibiotics. This method is more widely used since certain bacteria have evolved that are much more resistant to many treatemtns. Infections caused by Methicillin-Resistant Staphylococcus Aureus (MRSA) are exceptionally difficult to treat with regular broad-spectrum antibiotics.
Over time, some strains of staph bacteria have become resistant to treatment by penicillin-based antibiotics — because bacteria, like all life forms, evolves. People with MRSA infections have to take specialized medications, which are much harder on the body, in order to kill the bacteria. Overuse of any type of antibiotics can create resistant bacteria that are very difficult to fight. Therefore, physicians try to make sure to only use these medications when they are really necessary, in order to slow the evolution of antibiotic resistant bacteria.