A method for identifying a risk factor for diseases, disorders or conditions, such as those caused by human immunodeficiency virus, using the polymerase chain reaction and specific primers. Methods for treating patients having these diseases, disorders or conditions by antimicrobial treatment of the risk factor by combined antiviral and antibacterial treatment or by sustaining or stimulating the subject's immune system. Methods for screening biological products including red blood cell preparations. Primers and methods for detecting nucleic acids or microbial agents associated with red blood cells, such as those associated with red blood cells in subjects infected with HIV and undergoing antiretroviral therapy.