There are millions of bacteria on and within the body, some are potentially pathogenic (disease producing), some are outright pathogens, some are commensal and many are beneficial. However, most bacteria will cause disease if they colonize an area of the body where normally are not found. For example, a huge percentage of the human population are carriers of Staphylococcus aureus on the surface of their skin, causing little to no problems, but when the organism enters an open wound or the blood stream, a major life threatening situation may occur. The mouth is loaded with bacteria, again most will not cause any issue while they remain in the mouth, but introduced into an open wound or the blood stream an unintentional infection may occur.

The oral cavity as a reservoir of bacterial pathogens for focal infections

Pete