This term is applied here to members of animal kingdom only, which are protozoa (unicellular organisms), helminths (worms), and arthropods (insects, spiders and others). It is obvious that parasites are smaller organisms depending on a heterospecific (different type) organism for their living, either temporarily or permanently.


Types of parasites:

  1. According to the relation with the host’s body they are either:
    • Ectoparasite living on or outside the body (infestation).
    • Endoparasite living within the body (infection).
  2. According to the life history:
    • Facultative parasite, when it is capable of living outside the host and can also be parasitic.
    • Obligate parasite, when it is completely dependent on the host and cannot live outside its body.
    • Pseudoparasite, it is an artifact which could be mistaken as parasite during examination, like some pollen grains or yeasts.
    • Coprozoic is a foreign species that has passed through the alimentary tract without infecting the host, as some eggs of animal parasites or free living amoebae for example.
    • Erratic or ectopic parasite, which lives in the body of its host in another place than its normal one.
    • Opportunistic parasites, these are parasites which cause a self-limited infestation in healthy individuals but cause severe diseases to immunocompromised patients. 
  3. according to the time of parasitism:
    • Temporary parasite, living part of its life freely in the environment and seeks the host only to obtain nourishment.
    • Permanent parasite, remaining on or in the host from early life until maturity or its whole life.
  4. According to the number of the hosts:
    • Monoxenous, having only one host for its whole life cycle.
    • Heteroxenous, having several hosts, one for each stage during the life cycle of the parasite.Types of hosts:
    • According to the life cycle of the parasite the host is considered:
  1. Definitive or final host, this is the host that harbors the adult or sexual stage of the parasite.
  2. Intermediate host, in which part or all the larval stage or the asexual cycle takes place. Some parasites may need more than one intermediate host.
  3. Reservoir host, it is another animal than man which harbors the same parasite. It ensures the continuation of its life cycle and can act as a source of infection to man.
  4. Transfer host, it is an animal host that propagates the parasite carrying it from one place to another in or on its body.
  5. Incidental host, this is not the necessary host for the parasite’s survival or development.
  6. Vector, this is special name given to arthropods that share actively in the life cycle of parasites as intermediate or definitive or transfer hosts and help in infecting man and animals by them. The parasite is either endemic i.e. present in the same locality for a long period, or epizootic attacking many susceptible individuals in a new area in a short time. Parasitic infection and diseaseThe mode of transmission of infection varies from direct contact to a more complicated way through other vehicles e.g.Soil when polluted with human and animal excreta is infective with the previously mentioned stages as well as hook worm larvae.Arthropods can also mechanically transmit the parasites mentioned before on their body or through their gut and excreta from one place to another e.g. flies and roaches. Also they can take part in the life cycle and become thus a source of infection.Blood transfusion can be a method of transmission of blood parasites e.g. malarial parasites.Effects of parasite on the host:
  7. Parasites have many harmful effects on their hosts, which vary according to their pathogenesis, but generally they can be summarized as follows.
  8. Sexual intercourse can also transmit some parasites.  
  9. Congenital or connate transmission of the parasites from mother to foetus via placenta during pregnancy or during labour respectively.
  10. Air can also carry protozoan cysts and helminth eggs to susceptible hosts by inhalation or through uncovered food in slum areas with bad sanitation.
  11. Water which if polluted with human excreta can transmit protozoan cysts, helminth eggs and some larval stages as cercariae.
  12. Since parasitic infection often tends to run a chronic course with few symptoms, an infected individual may become carrier, so serving as a potential source of infection to the others. A carrier state represents the normal state of infection where there is an equilibrium between the host and the parasite.
  14. Geographic distribution:
  1. Mechanical effects, as pressure from an enlarging cyst, obstruction of a vessel or a hollow viscous.
  2. Invasion and destruction of the host cells by the parasite itself as in cases of malaria infecting R.B.Cs.
  3. Inflammatory reaction to the parasite or parasitic products.
  4. Facilitation of secondary infection by bacteria during entry of parasite through skin or bites of insects or even skin ulceration due to parasitic infection.
  5. Drain of host’s nourishment causing several deficiency manifestations, growth retardation and anemia.
  6. toxic effect of some parasitic products may affect the host e.g. bone marrow depression or some nervous manifestationsPrevention of parasitic infection: The measures taken for prevention of parasitic infection are:
  7. Proper knowledge about the life cycle and mode of infection and transmission of parasites during their existence outside the body helps in the control and prevention of disease.
  1. Treatment of infected individuals to reduce human sources of infection.
  2. Education in personal prophylaxis to prevent spread of infection and to reduce the chance of exposure.
  3. Sanitary control for food, water, living and working conditions and for waste disposal.
  4. Destruction or control for reservoir hosts and vectors to limit transmission of diseases.
  5. Application of biologic barriers to the transmission of parasites, this is an environment-friendly way to get rid of parasites without harmful effect on the environment by insecticides or such chemicals, e.g. by use of some types of fish that feed on mosquitoe larvae.