Infection: 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. An infection occurs when a microorganism enters a person’s body and causes harm.The microorganism uses that person’s body to sustain itself, reproduce, and colonize. These infectious microscopic organisms are known as pathogens, and they can multiply quickly. Examples of pathogens include: bacteria, viruses, fungi/ The immune system is an effective barrier against infectious agents. However, pathogens may sometimes overwhelm the immune system’s ability to fight them off. At this stage, an infection becomes harmful. Some pathogens have little effect at all. Others produce toxins or inflammatory substances that trigger negative responses from the body. This variation means that some infections are mild and barely noticeable, while others can be severe and life threatening. Some pathogens are resistant to treatment. Infection can spread in a variety of ways. Bacteria, viruses, fungi, and parasites are different types of pathogens.