Sometimes serious pathogens slip in because their surface is disguised as something the host cell walls can accept. That is how viruses work. Once inside a cell, their genetic material controls the cell. Infections like HIV get in this way, and then attack cells which are the basis of the immune system. Artificial means are often used to restore immune system function in an HIV-challenged body, and prevent the onset of AIDS. This is one of the most complex issues in immunology as it involves every level of that system. This research during the 1980s and 1990s radically changed the view of the human immune system and its functions and integration in the human body.