Immunodeficiency refers to a weakened or compromised immune system, which is unable to fight off infections effectively. An Immunodeficiency Virus (IDV) is a type of virus that attacks and destroys the body's immune cells, leading to an impaired ability to defend against diseases. This can result in increased susceptibility to illnesses and infections, as well as certain types of cancer. Examples of Immunodeficiency Viruses include HIV (Human Immunodeficiency Virus) which causes AIDS, and the Hepatitis C virus that attacks the liver's immune system.