MSM: Men who have sex with men.

MTCT: See Mother-to-child transmission.

Masturbation: Manual stimulation of the human sexual organs.

Maternal antibodies: Antibodies that have been passively acquired by an infant from the mother before birth. It is difficult to determine if an infant born to an HIV-positive mother also has the virus, as maternal antibodies to HIV continue to circulate in the infant’s blood up to the age of 15-18 months.

Microbicides: Agents that can block, kill, or immunize pathogens. Products are currently under development that could be used vaginally or rectally to block infection with HIV and other STIs.

Micro-organism: Any organism that can only be seen through a microscope, including protozoa, bacteria, fungi and viruses.

Monogamy: Having only one mate or partner at a time, or the practice of being married to only one person at a time.

Mother-to-child transmission (MTCT): The transmission of infection, typically HIV, from the mother to the child during pregnancy, labour, delivery or through breastfeeding.

Mucous membrane: Moist tissue lining the internal passages of the body, e.g. in the nose, mouth, vagina, and rectum.

Mutation: A change in a gene that results in new characteristics. HIV mutation causes varying levels of virulence.