CAUSES OF DISEASE
Diseases can be caused both by infective agents and by noninfectious causes including toxins, tumors, nutritional and metabolic problems traumas, and many others. Most diseases we address are caused by infective agents.
Infective agents most commonly are living organisms and typically are classified among five major kingdoms (Whittaker 1969). Viruses and prions are additional agents that, while not considered living entities, also are infective agents (Buchen-Osmond 2003).Living organisms with a parasitic lifestyle comprise the majority of species in the world; by one estimate parasites outnumber free-living species in a four to one ratio (Zimmer 2000). Among living organisms, an initial division typically is made between eukaryotic and prokaryotic organisms. Eukaryotic organisms all have double-stranded DNA enclosed within a nuclear membrane, an endoplasmic reticulum, mitochondria, Golgi apparatuses, lysosomes, and other cellular organelles; these organisms divide by mitosis or meiosis. Traditionally they have been distributed among the Kingdoms Protista (single-celled eukaryotes), Plantae, Ani- malia, and Fungi (Whittaker 1969) (Appendix 1).
In contrast, prokaryotic organisms lack true nuclei and have single-stranded DNA; they also lack such organelles as mitochondria, endoplasmic reticula, Golgi apparatuses, and lysosomes (Murray et al. 1999). Prokaryotes divide by fission rather than by mitosis or meiosis. Prokaryotes include both the Archaebacteria and Eubacteria (“true bacteria”) (Murray et al. 1999) and are classified in the Kingdom Monera (Whittaker 1969) (Appendix 1).
Viruses lie at the boundary between life and inert matter and are not typically included in classifications of living organisms, even though they regularly replicate, mutate, evolve, and serve as significant influences on the evolution of their hosts (Villarreal 2004).
Classification schemes have emerged for viruses based on their proposed evolutionary relationships (Buchen-Osmond 2003, van Regenmortel and Mahy 2004). Recently, considerable interest has emerged in the role of infectious proteins (“prions”), which have no nucleic acids (Buchen-Osmond 2003) but are important contributors to some wildlife diseases.Views about the evolutionary relationships within and between various groups have been changing (Doolittle 1999). We note some of the recent changes in proposed classifications (e.g., Adl et al. 2005); however, because of its long and well-established history, we base our discussions on the traditional five-kingdom system (Monera, Protista, Fungi, Animalia, and Plantae) (Whittaker 1969) in this text (Appendix 1). Viruses and prions are treated as addenda to the five-kingdom system. For each major group we provide a general definition of the group, a brief description of some of its distinctive features, and a summary of any recent taxonomic changes.
Although our main focus is on diseases caused by infective agents, there also is a wide variety of noninfectious diseases to which wildlife are subject (Fairbrother et al. 1996). Two that we will address include cancers and toxins.