The invasion and multiplication of microorganisms such as bacteria, viruses, and
parasites, that are not normally present within the body. An infection may cause no
symptoms and be subclinical, or it may cause symptoms and be clinically apparent. An
infection may remain localized, or it may spread through the blood or lymphatic vessels
to become systemic (bodywide). Microorganisms that live naturally in the body are not
considered infections. For example, bacteria that normally live within the mouth and
intestine are not infections.