Infection is the invading of the host organism’s body by another organism. It can be simply be broken down to in (inside) and felt (to feel); therefore infection is an internal phenomenon causing alteration in the normal body system of an organism (host) by another invading disease causing organism and therefore presenting clinical signs (symptoms) in the Host.
Parasite is an organism that lives, grows, feed, and sheltered in/on another organism (Host) contributing nothing to the survival of it host, and taking advantage of the generosity of the Host organism without making a useful return. This kind of relationship is termed parasitism; example is malaria parasite, hook worm, Ascaris etc
Parasitic infection is therefore invading of Host body by parasites which lives, feed, and sheltered in/on the Host. The parasites that live on (outside the host body) are called ECTOPARASITE and those that lives inside the host body are called ENDOPARASITE.
Pregnancy , however is fertilization and development of an ova (plural)/ovum (singular) forming fetus (offspring) or embryo in a female’s uterus. The common term use for pregnancy in human is GESTATION.
Pregnancy is formed by the coming together of male seed called SPERMATOZOA and the egg called OVA/OVUM from female individual resulting in embryo or fetus that develop in the uterus of the female. This present a physical and clinical manifestation (symptom) in female individual and therefore one begin to think and ask, “is pregnancy also an infection?’”
In an attempt to provide answer to the question, the following facts can be said of both parasite and pregnancy;
Parasite is an organism of one species living in an organism of other species (hetero specific relationship and driving its nourishment from the host, this is a parasitic relationship. Human embryo or fetus however is an organism of one species (homo-sapiens) living in the uterine cavity of an organism of the same species and deriving it nourishment from the mother. This is a dependent relationship.
A parasite is generally harmful to some degree to the host while human embryo developing in the uterine cavity does not usually cause harm to the mother, provided proper nutrition and care is maintained by the mother.
Human embryo or fetus make direct contact with the uterine lining of the mother for only a short period of time, it soon become isolated inside it own amniotic sac and make indirect contact with the mother only by ways of the umbilical cord and placenta. Parasite on the other hand makes direct contact with the host tissue often holding on by mouth parts, hooks, or suckers to the tissues involved (intestinal linings, lungs, connective tissue etc).
A parasite is an invading organism coming to parasitize the host from an outside source. Human fetus is formed from fertilized egg coming from outside source being formed in the ovary of the mother from where it may be fertilized to form zygote, the first cell of the new human being.
When parasite invades host tissue, the host tissue some times responds by forming a capsule of connective tissue to surround the parasite and cut it off from other surrounding tissue (phagocytosis). In pregnancy when the fetus attaches tissue and invades the lining tissue of the mother’s uterus the lining tissue responds by surrounding the human embryo but does not cut it off from the mother. Rather it establishes a means of close contact (the placenta) between the mother and the new human being.
Also, when parasite invades a host, the host usually responds by forming antibodies in response to the somatic antigens (molecules comprising the body of the parasite) or metabolic antigens (molecules secreted or excreted by the parasite). Parasitism usually involves an immunological response on the part of the host. In pregnancy mother does not to the presence of the embryo by producing humoral antibodies, but the trophoblast (the jacket of cells surrounding the embryo) blocks the action of these antibodies and therefore, the embryo is not rejected. The reaction is unique to the embryo-mother relationship.
In close, human embryo or fetus develops inside uterus of the mother for about nine month or more, deriving it nourishment from the mother. In spite of this it is not considered or treated to be a parasite.