"Immunosuppression"
            Immunosuppression is a term used by many people to describe the consequences of the formation of a defect in an existing immune system component; it is also popularly used to describe an inadequate/insufficient immune response.

    Hypersensitivity
            Hypersensitivity refers to an exaggerated immune response that may cause damage to a host's own tissues.  A hypersensitivity response is not an infrequent occurrence in some parasitic infections.

    Autoimmunity
            In some instances, parasitic infections may cause the host immune response to be directed against the host's own tissues.  Autoimmune damage can interfere with the development of a protective immune response against some parasites.

Host Inflammatory Response

    Nonspecific mechanisms/barriers
            Some components of the host inflammatory response (such as the kinin cascade, the clotting cascade, and the fibrinolytic system) are activated by almost any mechanical damage to host cells.  These barriers may interfere with the parasite's ability to survive, migrate, and reproduce in a host.

    Inflammatory processes linked to specific immune system components
            Some host cells associated with inflammation, such as eosinophils, are often controlled and directed (via cytokines/lymphokines, direct membrane to membrane contact) by activated, antigen-specific immune cells and/or antigen-specific antibodies.  These inflammatory processes may benefit the host's efforts to limit the presence of the parasite, yet in some cases these components may cause damage to the host's own tissues.

   Coexisting infections/other agents provoking inflammation
            The simultaneous presence of other infections and inflammation-triggering circumstances can influence the courses of some parasitic infections to the benefit of either the host or the parasite.

Constitutional Host Properties

    Anatomical/structural variation
            Host anatomical/structural patterns are a manifestation of the host's genotype.  Within a host species, there are probably anatomical patterns that favor the development of a parasite and other patterns that impede the development of the same species of parasite.

    Physiological variability
            Physiological differences between species, the physiological variation found in a population of members of the same species, and the variety of physiological states (such as pregnancy and lactation) possibly experienced by an individual during its lifetime may influence the development and outcome of parasitic disease in hosts.

    Age
            The course of several parasitic infections is significantly influenced by the age of the host.  Some of the effects of age on parasitic infections is probably related to the progressive development and gradual loss of acquired immunity to parasites as the host grows older; other factors not related directly to cell & humoral immunity, such as age-related changes in the composition of connective tissue matrix, may also affect the ability of a parasite to survive in a host.  As humans age, the acidity of gastric juice decreases, resulting in hypochlorhydria that may affect the ability to prevent some infectious agents from establishing infections in these hosts.

    Sex
            Typically, males in a host population are more susceptible than females to infection with certain parasites.  The increased susceptibility of males to certain parasites seems to be related to the physiological effects of testosterone on tissues that are not primarily involved in reproduction.  Reproductive cycles in females can have an effect on many parasitic infections.

    Genetics (results of either artificial and/or natural selection) 

Host Behavior

    Innate ("instinct")
        Patterns of "preprogrammed behavior that are present from birth are referred to as innate behavior, or instinct.

    Acquired ("learned")
        Most of the host behavior patterns that are linked to parasite transmission are learned behaviors.  These animal behaviors are acquired, via imprinting, habituation, etc., and the variations in these patterns may result in differences in parasite prevalences in populations of host species.

        Culturally-influenced
                Preferences that humans have for many features of their lifestyles are a consequence of acquired behaviors linked to the culture in which they grew up.  Food preparation, sexual practices, and recreational choices are culturally-influenced aspects of host behavior that can have a significant effect upon exposure to parasitic organisms.

        Socioeconomically-influenced
                Some aspects of human lifestyles, such as access to potable water, adequate sanitation facilities, and hygienically-prepared food items, are greatly influenced by "class".

        Husbandry
                The practices used by humans to maintain domestic animals can have great impact on exposure to parasite transmission stages, which can have an effect on the development of protective immunity and/or may result in the accumulation of an overwhelming number of parasitic organisms in a host.

        Incarceration
                Exposure to some parasitic infections is facilitated when humans are housed together under conditions that typically exist in prisons, jails, concentration camps, refugee camps, etc. 

        Mental illness/institutionalization
                There are parasitic infections that are more frequently observed in humans who suffer from various forms of mental illness than in people who are not thus affected.  Parasitism in the mentally ill often results from the ingestion of fecal material, soil, invertebrates, etc. that can be rich in parasite transmission stages.  

Biology of the Parasite

        "Virulence"
                Virulence, the ability of an organism to produce disease in a host, is probably a combination of the factors listed below.

        Parasite longevity

        Infective dose

        Size of the parasite population in the host
               A larger parasite burden in a host can cause greater amounts of structural damage, compete with the host for larger quantities of resources, and trigger more massive amounts of inflammation in many parasitic relationships.

        Stage of development in the host
                Some developmental stages of certain parasitic species are more likely than other developmental   stages of the same parasite species to damage host tissues, provoke inflammation, etc., thus resulting in morbidity in hosts.  

        Organ/tissue/cell location of the parasite in the host
                Some sites within the host are more sensitive to the presence of certain parasites than are other sites.  An active parasitic form in the CNS is usually a greater threat to the survival of a host than is the presence of a parasite in skeletal muscle.

         Route of transmission/developmental pattern/reproductive potential
                Parasites that have the ability to increase their numbers significantly in a rapid fashion within the tissues of hosts (disrupting normal cellular architecture, provoking inflammation, etc.) are more frequently associated with clinical illness than are those parasitic organisms that slowly produce smaller numbers of organisms that must exit the host in order to develop to an infective stage.

        Mode of parasite feeding; parasite nutritional requirements
                Several parasites feed in ways that damage the host more than other modes of parasite feeding.  A parasite that feeds by absorbing nutrient materials across its body surface is not directly causing mechanical damage to the host's tissues, while an organism that actively feeds on host tissues is causing demonstrable change in the normal architecture of host tissues.  The specific nutritional requirements of parasites may directly affect the course of parasitism or may affect other aspects of the host-parasite relationship that can influence the course of a parasitic infection.