Both viruses and bacteria can enter the body in different ways.You can eat them if they are in a certain type of food or live in the animal or plant you are eating.You can get them by drinking water also.You can breath them in from the air and they enter your lungs.If you touch a bacteria or virus and then rub your eyes or nose they can enter your body.
Once the bacteria or virus is in it can travel in the blood to any cell in your body.