Transmission electron micrograph (TEM) of mature virions of the virus causing AIDS (Acquired Immune Deficiency Syndrome). The virus designated HIV (Human Immunodeficiency Virus) belongs to the class of retroviruses. It affects T-lymphocytes of the immune system, particularly the T-helper cells responsible for stimulating B-lymphocytes to produce antibodies. Without this immune response the body's natural defence system is undermined and it falls prey to opportunistic infections, notably pneumonia and Kaposi's sarcoma. The virus is transmitted by sexual contact and through contaminated blood products. Magnification: x200,000 at 8x10 inch size.