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Acquired immune deficiency syndrome (AIDS) was first recognized in 1981 as an unprecedented group of opportunistic infections appearing in individuals with no known predisposition for immune system dysfunction (Durack, 1981).

The aetiological agents of AIDS, human immunodeficiency virus type 1 and 2 (HIV-1 and HIV-2), are retroviruses that probably originated in chimpanzees (Pan troglodytes) and the sooty mangabey monkey (Cercocebus atys), respectively (Gao et al., 1999; Hahn et al., 2000). HIV was isolated in 1983 and it was associated with AIDS in 1984 (Barre-Sinoussi et al., 1983; Gallo et al., 1984; Levy et al., 1984).

An estimated 42 million individuals are currently infected worldwide, with the vast majority of those in sub-Saharan Africa. Spread of HIV infection continued at an alarming rate in 2002 with over 5 million new infections and 3 million deaths (UNAIDS, 2002).

AIDS can be considered a disease of an immune system that is not able to function effectively. HIV can weaken the immune system to the point that the infected host is unable to control certain organisms that are usually controlled by a healthy immune system. On that account, these types of pathogens are known as "opportunistic" because they are able to express their pathogenic effect only in association with predisposing factors that weaken the immune system.

Some of these infections are well-recognized zoonoses that are naturally transmitted between vertebrates and humans. Others are associated with, but not directly transmitted by, animals. Aside from these direct encounters with animals, indirect contact may also occur through such means as contaminated food and water.

Interactions between animals and humans are truly complex and health care providers should be aware of the potential role of animals in infectious diseases of HIV infected patients. This relationship is not limited to direct contact between humans and animals, but could also be indirect. In the case of indirect transmission, complicated patterns are usually involved and knowledge of these is required to understand better the epidemiology of HIV related infections and to provide medical support to infected patients.

Treatments able to slow down the rate at which HIV weakens the immune system are now available as well as effective drugs to treat opportunistic pathogens. Nevertheless, there is a pressing need to better clarify the intimate correlation between host and opportunistic pathogens. This will give insights into the pathogenic pathways and provide scientific communities with better strategies to prevent, or at least reduce the rate of opportunistic infections which occur in HIV infected patients.

The aim of this guideline is to outline the most important zoonoses that play a significant role in the epidemiology of AIDS and to provide a practical and manageable tool for health workers involved in the care of HIV infected humans.

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