The best protection from spreading any disease is probably basic hygiene. Washing your hands after defecating or urinating is a great way to stop the spread of disease. Also, disposing of your garbage and waste is another way. Aside from the obvious, vaccination is another good way. Contagious diseases are often spread very easily. Becoming immune to them will stop them from being transmitted. To stop the spread of STDs, the best protection is to use protection while having intercourse.
The respiratory system brings oxygen into the lungs when you breathe. The digestive system breaks food down into nutrients such as glucose. Now the circulatory system enters the picture. It transports glucose and other nutrients from the digestive system to the cells.