The spectrum of zoonotic illness varies from skin eruptions or mild, self-limiting infections easily misdiagnosed as human influenza to serious, life-threatening disease. Some zoonoses can affect healthy people, while others are primarily found in individuals with debilitating illnesses and other conditions that compromise immunity. Zoonoses that are mild or asymptomatic in healthy hosts can be serious illnesses or have unusual presentations in those who are immunocompromised. In some cases, a suppressed immune response may also slow diagnosis if common tests rely on serology.

Primary immunodeficiencies, which are congenital defects, may affect humoral or cell-mediated immunity, or both. Some primary immunodeficiencies increase susceptibility to a single category of pathogens, while others broadly suppress defenses. Some may remain unnoticed except as an unusual susceptibility to certain illnesses, while others are obvious from infancy. Secondary immunodeficiencies can be caused by any acquired condition that compromises the immune system. Examples include splenectomy, diseases that affect metabolism (eg, diabetes), illnesses such as cancer that result in generalized debilitation, and infections such as malaria or AIDS. Some illnesses, such as chronic lung disease, can increase susceptibility by affecting innate (nonspecific) defenses. Injuries and burns can compromise the skin defenses that prevent pathogens from entering the body, as can indwelling catheters and implanted medical devices. Drugs can suppress immunity either deliberately (eg, drugs used to treat autoimmune diseases or prevent rejection in organ transplant patients) or as an adverse effect. Some drugs used in cancer chemotherapy are highly immunosuppressive. (Also see Immunologic Diseases.)

Physiologic states can also affect immunity. The immune system is relatively immature in newborns and young children, and it declines in the elderly. Pregnancy may result in risks to the mother, the fetus, or both. For example, hepatitis E has a fatality rate of ~1% in the general population but a 20% case fatality rate in pregnant women. Other pathogens, such as Toxoplasma gondii, may severely damage the fetus but can remain mild in the mother.

Last full review/revision March 2012 by James A. Roth, DVM, PhD, DACVM; Anna Rovid Spickler, DVM, PhD