AIDS is the acronym for Acquired Immune Deficiency Syndrome:
Acquired means people can get it (in this case, "it" is an infection);
Immune Deficiency means a weakness in the body's immune system;
Syndrome means a group of health problems that make up a disease.
AIDS is caused by a retrovirus (RNA-based virus) called Human Immunodeficiency Virus Type 1 (HIV-1). If a person gets infected with HIV, his/her body will try to fight the infection by eliciting an immune response that includes also the production of HIV-specific antibodies. Antibodies are special molecules produced by the immune system to fight diseases. AIDS may also be caused by the HIV Type 2 virus (HIV-2), but this virus is less virulent (infective) than HIV and it is not as prevalent (infective) as is HIV-1.
Antibodies are disease-specific. HIV-specific antibodies are produced when a person's body is infected with HIV. To find out if a person is infected with HIV, a blood test is done to look for HIV-specific antibodies. If a person has HIV-specific antibodies in his/her blood, this means that the person has been infected with HIV.
A person with HIV antibodies is defined as "HIV-positive." Being HIV-positive does not mean to have AIDS. Many people are HIV-positive but don't get AIDS for many years.
As the HIV disease progresses, it weakens the immune system. This makes the person more susceptible to opportunistic infections by other viruses, parasites, fungi and bacteria. In a healthy person, opportunistic infections usually aren't problematic. But in a person with a weakened immune system, these infections can cause severe problems.