Write about AIDS in Africa. No less than 250 words.

HIV/AIDS originated in Africa in the early 20th century and is a major public health concern and cause of death in many African countries. AIDS rates vary dramatically although the majority of cases are concentrated in Southern Africa. Although the continent is home to about 15.2 percent of the world's population,[1] more than two-thirds of the total infected worldwide – some 35 million people – were Africans, of whom 15 million have already died.[2] Sub-Saharan Africa alone accounted for an estimated 69 percent of all people living with HIV[3] and 70 percent of all AIDS deaths in 2011.[4] In the countries of sub-Saharan Africa most affected, AIDS has raised death rates and lowered life expectancy among adults between the ages of 20 and 49 by about twenty years.[2] Furthermore, the life expectancy in many parts of Africa is declining, largely as a result of the HIV/AIDS epidemic with life-expectancy in some countries reaching as low as thirty-four years.[5]