HIV/AIDS multiplies itself inside human cells. When the HIV virus enters the body it recognises CD4 cells (your immunity cells) and combines with them. The virus' genetic material (DNA) then combines with the human DNA. The virus then uses the human cell to multiply itself. These new virus cells find new CD4 cells to mix with and this process continues. When the HIV virus mixes with your immunity cells (CD4) it destroys them, which is why when you have HIV your immune system is weak and you get sick easily.
Attribution: Straight Talk Foundation