context of the environment and the biosphere. That is, host-parasite interactions must be considered in an ecological context and on an evolutionary basis for both the parasite and the host. Hosts and their parasites interact in ways that can be symbiotic and mutually beneficial at one extreme or deleterious and lethal at the other. In medical and public health parasitology, most of the attention is understandably focused on parasite-host interactions that are debilitating or lethal to the host and on understanding the molecular, biochemical, physiological, and immunological aspects of host-parasite relationships with the goal of designing and implementing prevention and control measures.
The mechanisms by which parasites cause infection and disease are diverse and complex. The diversity and complexity of the ecology of human parasites and their ability to cause infection and disease constitute a sufficiently substantive area of science for entire books to have been devoted to the subject (e.g., Bogitsh and Cheng, 1998; Bush et al., 2001; Gilles, 2000; Scott and Smith, 1994). As summarized by those authorities, some of the main factors responsible for the pathogenicity and virulence of parasites are (1) direct mechanical effects, (2) biochemical effects, (3) and immunological effects. Not only are the known mechanisms of parasite virulence and pathogenicity diverse and poorly understood, but new mechanisms and factors continue to be discovered or become more fully recognized for their importance. Because of the importance of molecular, biochemical, and immunological factors, genomics and proteomics (the study of all proteins produced by an organism) are contributing greatly to elucidation of the mechanisms of pathogenesis, virulence, and host susceptibility of waterborne and other pathogens. However, the biochemical mechanisms and genetic basis of pathology and virulence are far from being known for the vast majority of parasites and are unlikely to be fully elucidated and quantified for many of them for quite some time.
The protozoa are an ancient group of unicellular organisms (single-celled eukaryotes sized 3-30 μm) probably derived from unicellular algae, but most have subsequently lost their photosynthetic capabilities. Movement is accomplished through one of three modes: flagellae, ameboid locomotion, or cilia (Allen, 1987; Stossel, 1994). Although there are numerous free-living protozoa, some can be obligate parasites of humans as well as animals, are zoonotic (spread from animals to humans), and often spread through the fecal-to-oral route. As such, these are important organisms from a public health perspective and are associated with waterborne disease worldwide, including the United States (see also Chapters 1 and 2).