Scanning electron micrograph revealed the presence of the human immunodeficiency virus (HIV-1), (spherical in appearance), which had been co-cultivated with human lymphocytes. Note the lymphocyte in the lower left, and some of its extended pseudopodia. HIV-1 virions can be seen on the surface of this lymphocyte. A retrovirus, the Human Immunodeficiency Virus (HIV) was identified in 1983 as the pathogen responsible for the Acquired Immunodeficiency Syndrome (AIDS). AIDS is characterized by changes in the population of T-cell lymphocytes that play a key role in the immune defense system. In the infected individual, the virus causes a depletion of T-cells, called T-helper cells, which leaves these patients susceptible to opportunistic infections, and certain malignancies.