It is important to complete a course of antibiotics because even if you start to feel better, the infection may not have been killed, enabling it to either: 1. Build a defence against the prescribed antibiotic and come back stronger, meaning that you will then need to be prescribed a stronger course of antibiotics to fight the infection. Or 2. Remain dormant in your body with a new resilience against antibiotics. This increases the risk of you contracting MRSA, if you are hospitalised or come in to contact with the MRSA bug later in life, and with the resilience to antibiotics it will not be easily treated. Neither of the above are certain to happen if you do not complete a course of antibiotics, but both have a significantly high risk.