AIDS is an abbreviation of acquired immunodeficiency syndrome. This is a sexually-transmitted disease, caused by the HIV virus, which stands for human immunodeficiency virus. There are, nevertheless, other possible modes of transmission, including by blood, mother-to-child during childbirth
, and more. HIV attacks the immune system of the infected individual, greatly weakening the body even when faced with usually harmless diseases that should be fully controlled by the body's defense cells
. AIDS defines the entirety of symptoms observed in HIV infection progression. The AIDS stage is defined after a certain duration of infection when the body's defense cells are reduced to below a certain number, allowing for so-called "opportunistic" infections to attack the body.