The mouth plays host to many forms of micro-organisms (microbes). These microbes are mostly bacteria, but even in healthy patients, fungi and viruses may be found. Many of these microbes are harmless, but some are capable of causing infection or disease. These pathogenic (disease-causing) microbes are kept in check by the body’s natural defences or are prevented from growing excessively by the other microbes surrounding them. Any change in the balance of microbes in the mouth or a change in a person’s resistance to disease can allow pathogenic microbes to cause an infection. Fungal, bacterial, and viral infections can all occur. These infections are also discussed in Chap. 63 .