There is more bacteria in and on our bodies than there are cells in the human body. They live on the skin and in the bowel, and mostly are not contagious or harmful. They are essential for good health. But sometimes they, or more often bacteria from elsewhere, can cause infections.
Bacterial infections can affect the throat, the lungs, the skin, the bowel and many others parts of the body. Many are mild, some are severe. Treatment is usually a course of antibiotics (which don’t work against viruses).
Bacterial infections can be highly contagious and so you need to take special care to avoid spreading infections by washing hands, covering up when sneezing and coughing, and not sharing cups and drink bottles.