Immunodeficiencies caused by infectious agents may result from disruption of normal host barriers or dysregulation of cellular immunity, the latter serving to promote survival of the infectious agent through immune evasion. Such infections may be followed by opportunistic infections with a variety of other microorganisms. Classic infectious causes of immunodeficiency in companion animals are the immunodeficiency retroviruses, including feline immunodeficiency virus and feline leukemia virus. Other important causes include canine distemper virus; canine parvovirus 2; feline infectious peritonitis virus; rickettsial organisms that infect leukocytes; Leishmania; and fungal pathogens, such as Cryptococcus. Considerable research effort has been invested in understanding the mechanisms of pathogen-induced immunosuppression, with the hope that effective therapies may be developed that reverse the immunodeficiencies developed and in turn assist the host to clear persistent or life-threatening infectious diseases.