This chapter discusses on human immunodeficiency (HIV) infection and acquired immunodeficiency syndrome (AIDS) in question and answer format. More than 36.9 million individuals are worldwide sero-positive for HIV. AIDS is caused by the HIV which is a single stranded RNA human retrovirus and possesses the RNA-dependent DNA polymerase. HIV genome contains two main groups of genes and their products act as antigens i.e. standard genes and six regulatory/accessory genes. Transmission of HIV occurs when there is an exchange of blood or body fluids containing the virus or virus-infected cells. Virus usually enters the body through mucosal epithelia and clinical course can be divided into three main phases i.e. early acute phase, middle chronic phase, and final crisis phase. Natural history of HIV infection, important infections and presenting problems in AIDS, diagnosis of HIV infection or AIDS, and management of a patient with HIV infection are the import topics which are covered throughout this chapter.