Human immunodeficiency virus infection and acquired immune deficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV). It may also be referred to as HIV disease or HIV infection. Following initial infection, a person may not notice any symptoms or may experience a brief period of influenza-like illness. Typically, there is a prolonged period with no symptoms. As the infection progresses, it interferes more and more with the immune system, making the person much more susceptible to common infections, like tuberculosis, as well as opportunistic infections and tumors that do not usually affect people who have working immune systems. The late symptoms of the infection are referred to as AIDS. This stage is often complicated by an infection of the lung known as pneumocystis pneumonia, severe weight loss, skin lesions caused by Kaposi's sarcoma, or other AIDS-defining conditions.