Bacteria are a type of microscopic ‘bug’ of germ that can cause an infection. There are about 3 trillion bacteria that exist on or in the average human body without causing any disease. The gastrointestinal tract has the majority, followed by the skin surface and then the mouth (oral cavity). These bacteria do not cause an infection unless suitable conditions arise for it to invade tissue. Some bacteria are pathogenic and once it makes contact with the body, it rapidly causes an infection even in a person with an otherwise healthy immune system.