Infection: invasion of the “host” organism by “microorganisms” that can cause “pathological” conditions or “diseases.” (MeSH) 

Invasion and multiplication of “germs” in the body. Infections can occur in any part of the body and can spread throughout the body. The germs may be “bacteria,”  “viruses,” “yeast,” or “fungi.” They can cause a “fever” and other problems, depending on where the infection occurs. When the body's natural defense system is strong, it can often fight the germs and prevent infection. (NCIt) Invasion of the body by harmful organisms, such as bacteria, fungi, “protozoa,” or viruses. The infective agent may be transmitted by a patient or a carrier in airborne droplets expelled during coughing and sneezing, or by direct (physical) contact; by animal or insect “vectors,” by ingestion of contaminated food or drink, or from an infected mother to the “fetus” during “pregnancy” or birth. Organisms may invade via a wound or bite or through “mucous membranes.” After an “incubation” period symptoms appear, usually consisting of either localized “inflammation” and “pain” or more remote effects. Treatment with “antibiotics” is usually effective, but there are few specific treatments for many of the common “viral infections," including the common cold. (OxfordMed) Before the 19th-century innovations of anesthesia and 'antisepsis,' surgeons were severely limited in what they could do for their patients. Infections and shock limited their cutting to the extremities and superficial parts of the body. (Bruner, 215) Adjective - ‘infectious.’

Abscess: a localized collection of pus and “necrotic” tissue anywhere in the body, surrounded and walled off by damaged and inflamed tissues. The usual cause is a local “bacterial infection” that the body's defenses have failed to overcome. (OxfordMed) Abscesses occur when an area of tissue becomes infected and the body's immune system tries to fight it. White blood cells move through the walls of the “blood vessels” into the area of the infection and collect in the damaged tissue. During this process, pus forms. Abscesses can form in almost any part of the body. The skin, under the skin, and the teeth are the most common sites. Abscesses may be caused by bacteria, parasites, and foreign substances. (PubMedHealth2)

Boil: an abscess within the skin. (OxfordMed) An infection that affects groups of hair follicles and nearby skin tissue. (PubMedHealth2)

Bacterial Infection: of, pertaining to, or caused by bacteria. (Oxford) Infections and associated diseases by bacteria, “general” or “unspecified.” (NCIt)

Pus: a thick yellowish or greenish liquid formed at the site of an established infection. Contains dead white blood cells, both living and dead bacteria, and fragments of dead tissues. (OxfordMed) The buildup of fluid, living and dead white blood cells, dead tissue, and bacteria or other foreign substances. (PubMedHealth2)

Ulcer: a break in an “epithelial” surface. Also, a break in the skin extending to all its layers, or a break in the mucous membrane lining, that fails to heal and is often accompanied by inflammation. (OxfordMed) A lesion on the surface of the skin or a mucous surface, produced by the sloughing of inflammatory necrotic tissue. (NCIt)

Viral Infection: (infection) of, pertaining to, or caused by a “virus” or viruses. (Oxford) Any disease caused by a virus. (NCIt) A general term for diseases produced by viruses. (MeSH)