In all infectious disease, the following process or step must occur: encounter, the agent which is the micro-organisms must meet the host which is the sufferer. The micro-organisms must enter the host. The microbial agent must spread from the site of entry to other sites.
There must be damage done which could be due to the host immune response or the microbial agent or the both. There will be outcome, which in this case will be either the host win or the microbial agent win or both of them learn to live together. Over all, in each of these steps the host natural defences must be overcome.
Encounter: Our first encounter with micro-organisms is at birth. At birth, the baby comes in contact with micro organisms located in the mother’s vaginal canal and on her skin. Inside the womb, we were sterile. It is believe that the mothers’ blood carries only small numbers of infectious agent and is occasionally.
Even the placenta is a strong barrier to the passage of micro organisms to the fetus. However it is possible for the unborn baby to be infected through the placenta. A good example of these is the case of rubella also called congenital infections and syphilis as well as those caused by human immunodeficiency virus (HIV) or cytomegalovirus (CMV).
Disease organisms are contracted in two major ways namely: exogenously and endogenously.
Exogenously acquired disease: The disease are acquired as a result of our encounter with the infectious agents in the environment. There are numerous ways by which we could be infected through the environments, such as food, water, objects, insects bites, as well as from human or animal that we share our environment with. We human transmit easily many agents among ourselves through exchange of body fluids. Examples are like sneezing, touching, or sexual intercourse.
The way an infectious agent is acquired often suggest its’ mode of prevention. It has been possible to prevent many serious epidemics, especially in the developed parts of the world. Apart from vaccination, the preventive measure applied involve adequate sanitation, and improving the standard of living.
Endogenously acquired disease: These are disease gotten from micro organisms present on the skin and inside the body. Normally these micro organisms also called normal flora don’t cause any problem to the body, but when gained access into the body let say by cut can cause serious disease. A good example is staphylococci which lives on skin surface but when inside the deeper tissue produce pus.
It is still difficult to clearly demarcate which infection is exogenous and which infection is endogenous, so this term must be use very carefully. The problem here is that is difficult to precisely clarify what organisms constitute the normal flora.
Entry: This term can be explain in two ways, the ingress of micro organisms that are connected to the outside environment, or the entry of micro organisms into the inner tissue after crossing the epithelial barrier.
Ingress (entry without crossing epithelial barriers): Micro organisms enter the intestine by us swallowing them, and the lung by inhalation. Environmental micro organisms can also enter the urinary tractor the genital system. Micro organisms when inhale are in aerosol droplets or dust particle in the air we breathe. Also when we eat contaminated food and drink contaminated water we ingest micro-organisms into the intestine.
Note that micro organisms need not enter into the tissue before causing serious disease. Example of serious disease that occur without bacterial penetrating the epithelial surface are cholera, whooping cough, as well as the infection of the urinary bladder.
Penetration (entry into tissues after crossing epithelial barrier): some micro organisms pass directly through the epithelial, especially for single layer cell while most microbial agents need to be carried cross by insect bite or await breaks in the skin surface. For protozoa and worms that lives in insects that might be part of their life cycle. Insects are capable of spreading disease by carrying micro organisms on there their surface and by contaminating food stuff or the skin of a person. A good example is reduviid bugs which defecate the same time they bite.
However certain worms can burrow without help through the skin and cause infection. Examples of such are hookworms, which we can acquire when we walk on contaminated soil with barefoot.
Micro organisms can penetrate through deeper tissue by organ transplant or blood transfusions. Kidney transplant sometimes result in the infections by CMV, may be because the virus lives in the transplanted kidney. Other infectious agent that may be transmitted by blood transfusions are hepatitis b (HBV), degenerated disease of the central nervous system (Creutzfeldt-Jakob disease), and HIV, which is of the greater concern.
