Human Immunodeficiency Virus (HIV): infected T-lymphocyte (blue-green) showing virus particles (red) of typical dense core. HIV is the causative agent of Acquired Immune Deficiency Syndrome (AIDS). Retroviruses (such as HIV) produce the enzyme reerse transcriptase, which allows transcription of the viral genome onto the DNA of the host cell. When the host cell lyses, many virus particles are released for further infections. Magnification = 84,000x.