Updated: 2007
MAC
See
Mycobacterium Avium Complex (MAC). Macrophage
A large immune cell that destroys invading infectious agents. Macrophages can hide large quantities of HIV without being killed, acting as reservoirs of the virus. Macrophage-Tropic Virus
HIV strains that infect macrophages. They readily fuse with cells that have both CD4 and CCR5 molecules on their surfaces, whereas the same viral isolates fail to fuse with cells expressing only CD4. These isolates are the main ones found in patients during the symptom-free stage of HIV disease. Magnetic Resonance Imaging (MRI)
A noninvasive, non-x-ray diagnostic technique that provides computer-generated images of the body's internal tissues and organs. MAI
See
Mycobacterium Avium Complex (MAC). Maintenance Therapy
Also referred to as secondary prophylaxis. A therapy that prevents reoccurrence of an infection that has been brought under control. Malabsorption Syndrome
Decreased intestinal absorption of foods and nutrients resulting in loss of appetite, muscle pain, and weight loss. See
AIDS Wasting Syndrome. Malaise
A generalized, nonspecific feeling of discomfort or tiredness. Malaria
An infectious disease characterized by cycles of chills, fever, and sweating, caused by a parasite transmitted by a host mosquito. Malignant
Refers to cells or tumors growing in an uncontrolled fashion. Such growths may spread to and disrupt nearby normal tissue, or reach distant sites via the bloodstream. By definition, cancers are always malignant, and the term "malignancy" implies cancer. See
Metastasis. Mast Cell
A granulocyte found in tissue. The contents of the mast cells, along with those of basophils, are responsible for the symptoms of allergy. Mastitis
An inflammation of the breast. It is a condition that commonly results from inadequate or poor drainage of milk from the breast. Mastitis can be infective or noninfective in origin. Noninfective mastitis, which is most common, is usually the result of a blocked milk duct, which causes inflammation of the breast tissue. Mastitis frequently affects only one breast (whereas engorgement often occurs bilaterally) and is characterized by hard swelling, severe pain, fever (24 hours or longer), and localized redness around the affected area. Other causes include infrequent feeds or ineffective suckling; breast trauma and tissue damage; and pressure on the breasts from clothes, fingers, or other sources that inhibit milk flow and cause milk stasis leading to breast tissue inflammation. Infective mastitis is the result of bacterial infection. Poor breast attachment causing nipple fissures is a common pathway to infectious mastitis. MCH
Maternal Child Health Meconium
The first stools of the newborn. Typically thick, viscous, sticky, and dark green; usually sterile and odorless. Memory T Cells
A subset of T lymphocytes that have been exposed to specific antigens and can then proliferate (i.e. reproduce) on subsequent immune system encounters with the same antigen. Meninges
Membranes surrounding the brain or spinal cord. Part of the so-called blood-brain barrier. See
Meningitis. Meningitis
An inflammation of the meninges (membranes surrounding the brain or spinal cord), which may be caused by a bacterium, fungus, or virus. See
Central Nervous System (CNS); Cryptococcal Meningitis. Metaanalysis
A quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions. Metabolism
The chemical changes in living cells by which energy is provided for vital processes and activities and new material is assimilated. Metastasis
The spread of a disease (e.g. cancer) from an original site to other sites in the body. Microbes
Microscopic living organisms, including bacteria, protozoa, viruses, and fungi. Microbicide
An agent (e.g. a chemical or antibiotic) that destroys microbes. Research is being carried out to evaluate the use of rectal and vaginal microbicides to inhibit the transmission of sexually transmitted diseases, including HIV. Micronutrients
Vitamins or minerals that are necessary to maintain health and that the body must obtain from outside sources. Microsporidiosis
An intestinal infection that causes diarrhea and wasting in persons with HIV. It results from two species of microsporidia, a protozoal parasite. In HIV infection, it generally occurs when CD4+ T-cell counts fall below 100. See
AIDS Wasting Syndrome; Pathogen; Protozoa. Mitochondria
A rod-shaped or oval portion on the inside of a cell that produces most of the cell's energy. Mitochondrial Toxicity
Also referred to as mitochondrial dysfunction. A possible side effect of certain anti-HIV drugs, primarily NRTIs, that results in mitochondrial damage. This damage can cause symptoms in the heart, nerves, muscles, pancreas, kidney, and liver, and it can also cause changes in lab tests. Some of the common conditions related to mitochondrial toxicity are muscle and nerve disease and inflammation of the pancreas. Molluscum Contagiosum
A disease of the skin and mucous membranes caused by a poxvirus (molluscum contagiosum virus, MCV) infection. It is characterized by pearly white or flesh-colored papules (bumps) on the face, neck, and genital region. In persons living with HIV, molluscum contagiosum is often a progressive disease, resistant to treatment. When CD4+ cell counts fall below 200, the lesions tend to proliferate and spread. Monocyte
A large white blood cell that ingests infectious agents or other cells and foreign particles. When a monocyte enters tissues, it develops into a macrophage. Monotherapy
Use of a single drug or therapy to treat a disease or condition. Monovalent Vaccine
A vaccine that is specific for only one antigen. Morbidity
The condition of being diseased or sick; also the incidence of disease or rate of sickness. MRI
See
Magnetic Resonance Imaging (MRI). MSM
Men Who Have Sex with Men; a term created to include those who do not identify as gay or bisexual. MTCT
Mother-to-child transmission (of HIV). Mucocutaneous
Anything that concerns or pertains to mucous membranes and the skin (e.g. mouth, eyes, vagina, lips, or anal area). Mucosa
See
Mucous Membrane. Mucosal Immunity
Resistance to infection across the mucous membranes. Dependent on immune cells and antibodies present in the lining of the urogenital tract, gastrointestinal tract, and other parts of the body exposed to the outside world. Mucous Membrane
Moist layer of tissue lining the digestive, respiratory, urinary, and reproductive tracts--all the body cavities with openings to the outside world except the ears. Multiple Drug-Resistant Tuberculosis (MDR-TB)
A strain of TB that does not respond to two or more standard anti-TB drugs. MDR-TB usually occurs when treatment is interrupted, thus allowing organisms in which mutations for drug resistance have occurred to proliferate. See
Tuberculosis (TB). Mutation
In biology, a sudden change in a gene or unit of hereditary material that results in a new inheritable characteristic. As related to HIV: During the course of HIV disease, mutated HIV strains may emerge in an infected individual. These mutated strains may differ widely in their ability to infect and kill different cell types, as well as in their rate of replication. Of course, HIV does not mutate into another type of virus. Myalgia
Diffuse muscle pain or tenderness, usually accompanied by malaise (vague feeling of discomfort or weakness). Mycobacterium
Any bacterium of the genus Mycobacterium or a closely related genus. Mycobacterium Avium Complex (MAC)
A common opportunistic infection caused by two very similar mycobacterial organisms, Mycobacterium avium and Mycobacterium intracellulare (MAI), found in soil and dust particles. A bacterial infection that can be localized (limited to a specific organ or area of the body) or disseminated throughout the body. It is a life-threatening disease, although new treatments offer promise for both prevention and treatment. MAC disease is extremely rare in persons who are not infected with HIV. It generally occurs when the CD4+ T-cell count falls below 50. Mycosis
Any disease caused by a fungus. Myelin
A substance that sheathes nerve cells, acting as an electric insulator that facilitates the conduction of nerve impulses. Myelopathy
Any disease of the spinal cord. Myelosuppression
Suppression of bone marrow activity, causing decreased production of red blood cells (anemia), white blood cells (leukopenia), or platelets (thrombocytopenia). Myelosuppression is a side effect of some drugs, such as AZT. Myelotoxic
Destructive to bone marrow. Myocardial
Refers to the heart's muscle mass. Myopathy
Progressive muscle weakness. Myopathy may arise as a toxic reaction to AZT or as a consequence of the HIV infection itself. | 
