|Finishing the course of antibiotics prescribed for you is really important. Even if you are feeling better there may still be bacteria present in your body & stopping taking the antibiotics before they are all killed means they have the potential then to rapidly multiply and cause you the same symptoms again. When you are taking an antibiotic there is a race between the effects of the drug and your immune system on the bacteria and the ability of the bacteria to multiply. The longer bacteria survive the more likely they are to become resistant to the drug. If you stop taking antibiotics before they & your immune system have eliminated the bacteria the remaining bacteria will multiply rapidly. This rapid multiplication can result in mistakes in their DNA which can lead to resistant bacteria. Please visit the following websites for more details.
http://parenting.ivillage.com/tp/tphealth/0,,3q7m,00.html (Fairly easy to understand)
http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fimmunology%2Fans_025.html (A bit more scientific)