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A - Z Index

glossary a-z index

M-Tropic Virus

Also Known As: R5-Tropic Virus

MacrophageAudio (En español)

A type of white blood cell that fights infection by ingesting foreign substances, such as microorganisms and dead cells. Macrophages also act as antigen-presenting cells to stimulate other immune cells to fight infection.

See Related Term(s):  Antigen-Presenting Cell, White Blood Cell

Magnetic Resonance Imaging (MRI) Audio (En español)

A noninvasive procedure that uses magnetic fields and radio waves to produce three-dimensional computerized images of areas inside the body.

Maintenance TherapyAudio (En español)

Also known as: Secondary Prophylaxis

Additional drugs or other treatment given to prevent a reoccurrence of a disease or infection after initial treatment has already controlled the disease or infection. Sometimes maintenance therapy may be lifelong.

See Related Term(s):  Primary Prophylaxis

Major Histocompatibility Complex (MHC) Audio (En español)

A group of molecules found on the surface of almost every cell in the body. Major histocompatibility complex (MHC) molecules present protein fragments (peptides) to immune cells. If the cells recognize the peptides as foreign, the body mounts an immune response. In humans, MHC molecules are called human leukocyte antigens (HLAs).

Malabsorption SyndromeAudio (En español)

A group of symptoms that occur when the small intestine cannot absorb nutrients properly. Symptoms include abdominal pain, bloating, gas, and diarrhea. Malabsorption syndrome may be caused by certain diseases, infections, or drugs.

MalaiseAudio (En español)

A general feeling of discomfort, illness, or lack of well-being. Malaise can occur as part of a hypersensitivity reaction to certain antiretroviral (ARV) drugs.

MalariaAudio (En español)

A potentially life-threatening disease caused by four strains of protozoa called Plasmodium. Malaria is spread by the bite of a mosquito infected with the protozoa. Malaria is endemic in parts of Asia, Africa, Central and South America, Oceania, and certain Caribbean islands. Symptoms of malaria can include extreme exhaustion, chills, profuse sweating, fever, vomiting, diarrhea, other flu-like symptoms, and jaundice. Both malaria and HIV cause substantial morbidity and mortality worldwide, particularly in sub-Saharan Africa. Recent evidence shows important interactions between the two diseases.

See Related Term(s):  Opportunistic Infection

MalignantAudio (En español)

Threatening to health or life, such as a malignant disease. Malignant may also refer to the uncontrolled, invasive growth of cells, such as in a malignant tumor.

Mast CellAudio (En español)

Also known as: Mastocyte

A type of white blood cell found in almost all tissues, particularly in the skin. Mast cells help the body fight infection by triggering an inflammatory response to an antigen.

See Related Term(s):  Antigen, White Blood Cell


Also Known As: Mast Cell

Maternal-Child Transmission

Also Known As: Mother-to-Child Transmission

MaturationAudio (En español)

The final and seventh step in the HIV life cycle. Maturation takes place after the virus has pushed itself out of the host cell. During maturation, protease (an HIV enzyme) releases certain viral proteins that allow HIV to mature (become infectious).

See Related Term(s):  Life Cycle, Protease, Protease Inhibitor

Medication Event Monitoring System (MEMS) Audio (En español)

A device used to monitor medication adherence. A medication event monitoring system (MEMS) monitor consists of a conventional medicine container fitted with a special closure that records the time and date of each time the container is opened and closed.

See Related Term(s):  Adherence

MedlineAudio (En español)

A federal database that contains references and summaries for biomedical and life science publications from around the world. Most of the publications are from scholarly journal articles. Some of the listings include a link to the free full text of the article. Medline is a service of the U.S. National Library of Medicine.

See Related Term(s):  National Library of Medicine

MedlinePlusAudio (En español)

A federal Web site that contains reliable and up-to-date information about diseases, conditions, and wellness issues for patients and their families and friends. The Web site is written in easy-to-understand language and includes videos and illustrations. MedlinePlus is a service of the U.S. National Library of Medicine.

See Related Term(s):  National Library of Medicine

Memory Cell

Also Known As: Memory Lymphocyte

Memory LymphocyteAudio (En español)

Also known as: Memory Cell

A type of lymphocyte. Memory lymphocytes can recognize an antigen introduced into the body during a prior infection or vaccination. Memory lymphocytes mount a rapid and strong immune response when exposed to an antigen for a second time. Both T lymphocytes (T cells) and B lymphocytes (B cells) can become memory cells.

See Related Term(s):  Antigen, B Lymphocyte, Lymphocyte, T Lymphocyte

MeningitisAudio (En español)

Inflammation of the meninges, which are three layers of tissue that cover and protect the brain and spinal cord. Meningitis may be caused by a bacterial, fungal, or viral infection; drug allergies; cancer; or other diseases. Symptoms may include fever, headache, vomiting, malaise, and stiff neck. If left untreated, meningitis can lead to convulsions, coma, and death.

Messenger RNA (mRNA) Audio (En español)

A type of RNA that carries the genetic information needed to make a protein.

See Related Term(s):  Protein, Ribonucleic Acid

MetabolicAudio (En español)

Pertaining to metabolism.

See Related Term(s):  Metabolism

Metabolic SyndromeAudio (En español)

Also known as: Syndrome X

A combination of risk factors that increase the risk for heart disease, diabetes, and stroke. To be diagnosed with metabolic syndrome, a person must have at least three of the following metabolic risk factors: high blood pressure, abdominal obesity, high triglyceride levels, low levels of high-density lipoprotein (HDL) cholesterol, or high fasting blood sugar levels. Use of some antiretroviral (ARV) drugs may cause or worsen risk factors associated with metabolic syndrome.

MetabolismAudio (En español)

All of the physical and chemical processes that produce or use energy within the body. Drug metabolism refers to the breakdown of drugs in the body.


Also Known As: Microorganism

MicrobicideAudio (En español)

A drug, chemical, or other substance used to kill microorganisms. Increasingly, the term is used specifically for substances that prevent or reduce the transmission of sexually transmitted infections, such as HIV.

See Related Term(s):  Microbicide, Microbicide Trials Network

Microbicide Trials Network (MTN) Audio (En español)

A federally funded clinical trials network. Many Micobicide Trials Network (MTN) trials focus on evaluating microbicides and other promising HIV prevention approaches. 

See Related Term(s):  Clinical Trial, Microbicide

MicroorganismAudio (En español)

Also known as: Microbe

An organism that can be seen only through a microscope. Microorganisms include bacteria, protozoans, and fungi. Although viruses are not considered living organisms, they are sometimes classified as microorganisms.

MicrosporidiosisAudio (En español)

An infection caused by several species of the protozoan parasite microsporidia. Microsporidia usually infect the gastrointestinal tract but may also infect other parts of the body, such as the eyes, respiratory tract, or brain. The most common symptoms are chronic diarrhea and wasting, but symptoms vary greatly depending on the site of infection. Microsporidiosis occurs primarily in people with weakened immune systems, especially people with HIV or people who have undergone organ transplants.

See Related Term(s):  Opportunistic Infection

MitochondriaAudio (En español)

A structural unit within cells that produces energy through a process called cellular respiration.

See Related Term(s):  Mitochondrial Toxicity

Mitochondrial ToxicityAudio (En español)

Damage to mitochondria. Mitochondrial toxicity may affect different parts of the body, including the heart, nerves, muscles, pancreas, kidneys, and liver. Conditions resulting from mitochondrial toxicity can include muscle weakness, inflammation of the pancreas (pancreatitis), high levels of lactic acid in the blood (lactic acidosis), changes in distribution and amount of body fat (lipodystrophy), and fatty liver (hepatic steatosis). Use of certain antiretroviral (ARV) drugs may cause mitochondrial toxicity.

See Related Term(s):  Mitochondria

Mixed Tropic Virus

Also Known As: Dual-Tropic Virus

Modified Directly Observed Therapy (m-DOT) Audio (En español)

A variation of directly observed therapy (DOT). Modified-DOT (m-DOT) is when a health care professional watches a person take some, but not all, medication doses.

Molluscum ContagiosumAudio (En español)

A common, usually mild skin disease caused by the virus Molluscum contagiosum and characterized by small white, pink, or flesh-colored bumps with a dimple in the center. Molluscum contagiosum is spread by touching the affected skin of an infected person or by touching a surface with the virus on it. The bumps can easily spread to other parts of the body if someone touches or scratches a bump and then touches another part of the body. Compared to people with healthy immune systems, people with weakened immune systems, including people with HIV, are at higher risk for molluscum contagiousum and are more likely to have bumps that are larger and more difficult to treat.

MonoinfectionAudio (En español)

When a person has only one infection.

See Related Term(s):  Coinfection

MononeuropathyAudio (En español)

Neuropathy that damages only one nerve, resulting in symptoms that are linked specifically to the affected nerve.

See Related Term(s):  Neuropathy

MonotherapyAudio (En español)

Using only one drug to treat an infection or disease. Currently, monotherapy for the treatment of HIV is not recommended outside of a clinical trial. The recommended treatment for HIV infection is combination antiretroviral therapy (ART), using a regimen that includes three or more antiretroviral (ARV) drugs from at least two different HIV drug classes.

See Related Term(s):  Antiretroviral Therapy

MorbidityAudio (En español)

Disease state or symptom. Morbidity rate is a measure of the frequency of occurrence of disease among a defined population during a specified time period.

MortalityAudio (En español)

The state of being mortal (subject to death). Mortality rate is a measure of the frequency of occurrence of death among a defined population during a specified time period.

Mother-to-Child Transmission (MTCT) Audio (En español)

Also known as: Maternal-Child Transmission, Perinatal Transmission

When an HIV-infected mother passes HIV to her infant during pregnancy, labor and delivery, or breastfeeding (through breast milk). Antiretroviral (ARV) drugs are given to HIV-infected women during pregnancy and to their infants after birth to reduce the risk of mother-to-child transmission (MTCT) of HIV.

See Related Term(s):  Prevention of Mother-to-Child Transmission


Also Known As: Magnetic Resonance Imaging


Also Known As: Messenger RNA

MSMAudio (En español)

Acronym for men who have sex with men.

MSMWAudio (En español)

Acronym for men who have sex with men and women.


Also Known As: Mother-to-Child Transmission


Also Known As: Microbicide Trials Network

MucocutaneousAudio (En español)

Pertaining to the mucous membranes and skin. Mucocutaneous areas of the body include the mouth, eyes, vagina, and anus.

Multicenter AIDS Cohort Study (MACS) Audio (En español)

Started in 1984, the study involves collection of biological specimens and medical and behavioral data on MSM (men who have sex with men) in order to study the natural and treated history of HIV. The Multicenter AIDS Cohort Study (MACS) has significantly contributed to the understanding of HIV, AIDS, and the effects of antiretroviral therapy (ART).

Multiple Drug Resistant Tuberculosis (MDR-TB) Audio (En español)

A form of tuberculosis (TB) that is resistant to the two most effective antibiotics commonly used to cure TB infection (isoniazid and rifampin). People infected with multiple drug resistant-TB (MDR-TB) are at high risk for treatment failure. They are also at risk for further drug resistance, which can lead to life-threatening disease—extensively drug resistant tuberculosis (XDR-TB).

See Related Term(s):  Drug Resistance, Extensively Drug Resistant Tuberculosis, Tuberculosis

Multivalent Vaccine

Also Known As: Polyvalent Vaccine

MutagenicAudio (En español)

Causing a mutation, which is a permanent change in the genetic material of a cell or microorganism. A mutation may alter a trait or characteristic of a person or may cause disease. Mutations can be inherited or can occur spontaneously. Before being approved, drugs—including antiretroviral (ARV) drugs—are tested to assess their potential for mutagenic effects.

See Related Term(s):  Mutation

MutationAudio (En español)

A permanent change in the genetic material of a cell or microorganism. Some mutations can be transmitted when the cell or microorganism replicates. Some HIV mutations cause the virus to become resistant to certain antiretroviral (ARV) drugs.

See Related Term(s):  Drug Resistance

MyalgiaAudio (En español)

Also known as: Myodynia, Myoneuroalgia

Muscle pain.

Mycobacterium Avium Complex (MAC) InfectionAudio (En español)

An infection caused by a group of bacteria called Mycobacterium avium complex (MAC). MAC include Mycobacterium avium, Mycobacterium intracellulare, and other similar Mycobacteria. MAC bacteria can be found in drinking water, dirt, and household dust. MAC infection usually starts in the lungs and intestines, but can spread throughout the body (disseminated). Symptoms of disseminated MAC infection include fever, night sweats, weight loss, abdominal pain, fatigue, and diarrhea. Most people with healthy immune systems are not affected by the bacteria. In people infected with HIV, MAC infection that is outside of the lungs (extrapulmonary) or that has disseminated is an AIDS-defining condition.

See Related Term(s):  AIDS-Defining Condition, Mycobacterium Avium-Intracellulare (MAI) Infection, Opportunistic Infection

Mycobacterium Avium-Intracellulare (MAI) InfectionAudio (En español)

An infection caused by two closely related and hard-to-distinguish bacteria, Mycobacterium avium and Mycobacterium intracellulare. These two bacteria can be found in drinking water, dirt, and household dust. Most people are not affected by the bacteria, but for people with severely weakened immune systems, the bacteria can cause infection. M. intracellulare tends to cause lung disease, and M. avium tends to spread throughout the body (disseminated). Symptoms of disseminated Mycobacterium avium-intracellulare (MAI) infection include fever, night sweats, weight loss, abdominal pain, fatigue, and diarrhea. In people infected with HIV, MAI infection that is outside of the lungs (extrapulmonary) or that has disseminated is an AIDS-defining condition.

See Related Term(s):  AIDS-Defining Condition, Mycobacterium Avium Complex (MAC) Infection, Opportunistic Infection

Mycobacterium Kansasii InfectionAudio (En español)

An infection caused by the bacteria Mycobacterium kansasii. The bacteria can be found in water, dirt, and food. M. kansasii usually infects the lungs, but can also infect other organs and/or can spread throughout the body (disseminated). Most people with healthy immune systems are not affected by the bacteria. In people infected with HIV, M. kansasii infection that is outside of the lungs (extrapulmonary) or that has disseminated is an AIDS-defining condition.

See Related Term(s):  AIDS-Defining Condition

Mycobacterium TuberculosisAudio (En español)

The bacterium that causes tuberculosis (TB). Mycobacterium tuberculosis usually infects the lungs, but it can also infect other parts of the body, such as the kidneys, spine, and brain. M. tuberculosis is spread when a person with an active infection coughs, sneezes, speaks, or sings, and then a person nearby breathes in the bacteria.

See Related Term(s):  Tuberculosis

MycosisAudio (En español)

An infection with or disease caused by a fungus.

See Related Term(s):  Fungus

MyelinAudio (En español)

A whitish, fatty substance that forms an insulating layer around nerves. Myelin helps nerve signals transmit quickly and efficiently along nerve cells.

MyeloablationAudio (En español)

Severe bone marrow suppression. Use of some antiretroviral (ARV) drugs may cause myeloablation.

See Related Term(s):  Bone Marrow, Myelosuppression

MyelosuppressionAudio (En español)

Also known as: Bone Marrow Suppression

Impaired bone marrow function. Myelosuppression reduces bone marrow production of red blood cells, white blood cells, and platelets. Use of certain antiretroviral (ARV) drugs may cause myelosuppression.

See Related Term(s):  Bone Marrow

Myocardial InfarctionAudio (En español)

A heart attack. Heart attacks occur when blood flow to the heart is reduced or interrupted, causing severe damage or death to the heart muscle (myocardium). Use of some antiretroviral (ARV) drugs may increase the risk for a myocardial infarction.


Also Known As: Myalgia


Also Known As: Myalgia

MyopathyAudio (En español)

A disease of muscle tissue. Use of certain antiretroviral (ARV) drugs may cause myopathy.