M-Tropic Virus
See:    R5-Tropic Virus

MAC
See: Mycobacterium Avium Complex

Macrophage
A type of disease-fighting white blood cell that destroys foreign invaders and stimulates other immune system cells to fight infection.

MACS
See: Multicenter AIDS Cohort Study

Magnetic Resonance Imaging (MRI)
A noninvasive technique that uses magnetic fields and radio waves instead of x-rays to produce 3-dimensional computerized images of the body's internal tissues and organs.

Maintenance Therapy
Also known as secondary prophylaxis. A treatment to prevent an infection from coming back after it has been brought under control.

Major Histocompatibility Complex (MHC)
See:    Human Leukocyte Antigen

Malabsorption Syndrome
A condition that occurs when the intestines have problems absorbing nutrients. Malabsorption syndrome is associated with HIV infection and can cause loss of appetite, muscle pain, and weight loss.
See Also:   Wasting Syndrome

Malaise
A general feeling of discomfort or not feeling well.

Malignant
Referring to uncontrolled cell growth that may spread to other tissue, such as in cancer.

MDR-TB
See: Multiple Drug-Resistant Tuberculosis

Medication Event Monitoring System (MEMS)
A method of measuring drug adherence that uses a computer chip embedded in a pill bottle lid to record the date and time each dose is taken. Current obstacles to its use include its large size, the possibility of malfunction when refrigerated, and inaccurate reporting when pillboxes are used in place of the original bottle.
See Also:   Adherence

MedlinePlus
A database of health information developed by the National Library of Medicine (NLM). MedlinePlus has information on several hundred diseases and conditions as well as other health information. http://www.nlm.nih.gov/medlineplus/

Memory T Cells
A specific type of infection-fighting T cell that can recognize foreign invaders that were encountered during a prior infection or vaccination. At a second encounter with the invader, memory T cells can reproduce to mount a faster and stronger immune response than the first time the immune system responded to the invader.
See Also:   T Cell

MEMS
See: Medication Event Monitoring System

Meningitis
Inflammation of the membranes surrounding the brain or spinal cord. Meningitis can be caused by a bacterium, fungus, or virus such as HIV.

Messenger RNA (mRNA)
A molecule that carries genetic instructions for building a particular protein from the cell's DNA to the place in a cell where proteins are assembled. There, the messenger RNA serves as a blueprint for the construction of a specific protein.
See Also:   Translation
                   Transcription

Metabolic Syndrome
Also known as Syndrome X. A cluster of disorders affecting the body's metabolism, including high blood pressure, high insulin levels, excess body weight, and abnormal cholesterol levels. Some anti-HIV drugs may cause or worsen these metabolic disorders.

Metabolism
The chemical reactions that produce energy for the body.

MHC
See: Major Histocompatibility Complex

Microbes
Living organisms that can be seen only through a microscope, including bacteria, protozoa, viruses, and fungi.

Microbicide
A natural or man-made substance that kills microbes. Researchers are studying the use of microbicides to prevent the transmission of sexually transmitted diseases (STDs), including HIV infection.
See Also:   Microbes

Microsporidiosis
An infection of the intestines caused by a parasite. The infection can cause diarrhea and wasting (loss of weight and strength) in people with HIV.

Mitochondria
Rod-like structures that produce energy for a cell.

Mitochondrial Toxicity
A condition in which mitochondria are damaged. This condition is a potential side effect of NRTIs and can cause problems in the heart, nerves, muscles, pancreas, kidneys, and liver.
See Also:   Mitochondria

Molluscum Contagiosum
A disease of the skin and mucous membranes caused by a virus. The condition causes pearly white or flesh-colored bumps on the face, neck, underarms, hands, and genital region. In people with HIV, molluscum contagiosum can get worse with time and often becomes resistant to treatment.

Morbidity
The rate of sickness or disease within a certain population.

Mortality
The death rate, measured as the number of deaths within a certain population. The measure can apply to death from a particular disease or condition.

Mother-to-Child Transmission (MTCT)
The passage of HIV from an HIV-infected mother to her infant. The infant may become infected while in the womb, during labor and delivery, or through breastfeeding.

MRI
See: Magnetic Resonance Imaging

mRNA
See: Messenger RNA

MTCT
See: Mother-to-Child Transmission

Mucocutaneous
Relating to mucous membranes and the skin (for example, the mouth, lips, eyes, vagina, or anal area).

Multicenter AIDS Cohort Study (MACS)
An ongoing study of HIV infection in men. The study is co-funded by the National Institute of Allergy and Infectious Diseases (NIAID); the National Cancer Institute (NCI); and the National Heart, Lung, and Blood Institute (NHLBI) and is conducted at four clinical centers. Information about the natural history of HIV disease, the impact of treatment on disease progression, the role of genetic factors, and other long-term therapy issues are continually reported from study evaluations.

Multiple Drug-Resistant Tuberculosis (MDR-TB)
A tuberculosis (TB) infection that does not respond to two or more standard anti-TB drugs. MDR-TB usually occurs when inadequate or improper treatment allows the bacteria that cause TB to continue multiplying and become drug resistant.
See Also:   Tuberculosis

Mutation
A change or adaptation that can be passed down to future generations. Mutations can occur only when a virus is actively replicating, and not when anti-HIV drugs have suppressed the viral load to undetectable. If HIV replication is not well controlled, an individual's original HIV strain can adapt to infect different cell types or resist different anti-HIV drugs.

Myalgia
Muscle pain or tenderness that spreads throughout the body and is usually accompanied by a general feeling of discomfort or weakness.

Mycobacterium Avium Complex (MAC)
An infection caused by two bacteria found in soil and dust particles. The infection can be limited to a specific area or can spread throughout the body. This life-threatening disease is extremely rare in people who are not infected with HIV, and MAC is considered an AIDS-defining condition in HIV-infected people.

Mycosis
Any disease caused by a fungus.

Myelosuppression
Decreased bone marrow function that results in reduced production of red blood cells, white blood cells, or platelets. Myelosuppression is a side effect of certain anti-HIV drugs.

Myopathy
A disease of muscle tissue. Myopathy may be caused by certain anti-HIV drugs or as a consequence of HIV infection itself.