AIDS is the abbreviation1) of ACQUIRED IMMUNE DEFICIENCY SYNDROME．It is a complicated illness that may involve several phases．it is caused by a virus that can be passed from person to person．AIDS impairs the human body's immune system，the system responsible for keeping off disease，and leaves the victim easily affected by various infection．

　　The virus enters the bloodstream and destroys certain white blood cells，called T lymphocytes2)，which play a key role in the functioning of the immune system．The virus can also infect other types of cells in the body，including the immune－system cells knows as macrophages3)．Unlike T lymphocytes，however，macrophages are not killed by the virus．

　　Most people recently infected by the AIDS virus look and feel healthy．In some people the virus may remain inactive，and these people act as carriers，remaining apparently healthy but still able to infect others．After a few years，some people may develop AIDS－related complex，or ARC．Its symptoms may include fever，fatigue4)，weight loss，skin rashes，a fungal5) infection of the mouth known as thrush，lack of resistance to infection，and swollen lymph nodes．

　　AIDS is spread by direct infection of the bloodstream with body fluids that contain the AIDS virus，particular blood and semen from an HIV－infected person．The virus is usually spread through various forms of sexual intercourse，the transfusion of virus－infected blood，or the sharing of HIV－contaminated intravenous6) need les．