The body's internal defense mechanism consists of the White Blood Cells or the WBCs. These are our body's internal soldiers who fight against bacteria, virus and other forms of infection that may affect us. In areas where the WBCs are fighting and are being killed by these invaders, fresh WBCs are supplied by the circulatory system. Once back up arrives in the necessary areas, our WBC would continue to defend our body from such infections.
Even if our WBC fail to prevent the infection and the virus/bacteria affects us (Like a common flu) when you take medicines, these medicines would get mixed in our blood and would be circulated to all parts of the body to ensure that the medicine along with our WBCs (who are badly waiting for strong back up and ammunition) would kill the bacteria or virus and help us recover from the ailment.