HIV is a virus that uses the genetic material from our CD4 cells to make more copies of itself. The virus likes one particular cell in our immune system called a CD4 cell or t-cell. When it uses that cell's genetic material, it damages the T-cell making it unable to do its job in our immune system. The more of these CD4 cells that are damaged, the weaker your immune system becomes. Eventually, your immune system will become so weak that it will not be able to protect you from other illnesses and infections, thus you become sick. Simply put, HIV doesn't make you sick. It weakens your immune system, allowing other illnesses and infections to make you sick.