To establish an infectious disease, a disease agent (including pathogenic bacteria, fungi, viruses and protozoa) must first come in contact with a susceptible human host. This phase is called contact or encounter. ‘Humans’ first encounter with microorganisms (either pathogenic or non-pathogenic microbes) starts immediately after birth, and this marks the period in which the newborn begin to build up its own normal flora. But as the infant grows, he or she can come in contact with varying microorganisms” (both harmless and pathogenic organisms) in the environment that are likely to cause an infection. Adults can encounter pathogens from different routes or sources including but not limited to the air, food, water, fomites or via sexual intercourse and other body-to-body contacts, as well as contacts with animals or plants.


After a successful host-microbe encounter or interaction, pathogens begin to find their way into the host’s body via natural openings on the body (e.g., mouth, nose, ears, and vagina) or through wounds, cuts or abrasions on the skin. This phase is called invasion. Pathogenic microbes can also penetrate deeper tissues of the body via insect bites or through infected sharp objects that pierces the skin. Upon entry into the body, the infectious disease agents begin to find their way to other specific cells, organs and tissues far away from their point of entry. This phase is called spreading. Pathogenic microbes after spreading to their specific and target organs or tissues in the host, must be able to undergo reproduction in order to increase the number of infectious particles to a level that is necessary to establish an infection. This stage is called multiplication. At this stage, the invading microbe increases their infectious load.

Clinical signs and symptoms of a particular infectious process are not usually noticeable upon entry of the pathogen into the body until the infectious or microbial load in the host has reached the height or peak that is considered necessary to establish a disease condition. It is worthy of note that there can still be some variations in some disease processes where clinical signs and symptoms only appear after a given period of time (i.e., when the infectious agent is still at its window period). Also, some infectious disease agents produce toxins upon entering their hosts, and in such instances may not necessarily need to undergo multiplication before a disease or infection is established. Infectious disease agents multiply successfully in the host through a variety of mechanisms including disruption of the host’s immune system, production of toxins and environmental conditions of the body.

Damage of the host’s organ(s) is the next step that follows multiplication of a pathogenic microbe in vivo. Pathogens have the potential to damage and stop the normal function of one or more organs or tissues of their host, and this leads to a full blown infection or disease that may warrant hospital visit and medical treatment. Some disease conditions when not properly treated either therapeutically or via surgery can be wasting in nature i.e., it can be a dead end in which the host eventually dies. If the host survives the pathogen invasion, both host and pathogen can coexist and continue to live together, and the asymptomatic individual can be a reservoir or source via which the pathogenic microbe can infect a new susceptible host. This is only possible if available therapeutic options coupled to the host’s immune system are not able to eradicate or completely flush out the pathogen from the body of the individual.

Host immune response (especially violent responses) to the body’s invasion by a pathogenic microbe can sometimes lead to catastrophe in which some organs or tissues become affected. This can be seen in autoimmunity, in which the host’s immune response is directed against self or innate antigens of the body instead of the invading pathogen. Pathogenic microorganisms that cause diseases in humans may be endogenous (i.e., they are part of the normal flora of the body) or exogenous (i.e., they are pathogenic microbes that penetrate the body from the outside). Endogenous microorganisms can initiate an infection when the body’s immune system is compromised or when resident microflora becomes transient i.e., moved from its local position to a new location in the body. On the other hand, exogenous microorganisms are often acquired from the external environment such as from food, water, air or from inanimate objects and even from infected individuals.      

Further reading

Brooks G.F., Butel J.S and Morse S.A (2004). Medical Microbiology, 23rd edition. McGraw Hill Publishers. USA.

Gilligan P.H, Shapiro D.S and Miller M.B (2014). Cases in Medical Microbiology and Infectious Diseases. Third edition. American Society of Microbiology Press, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Mahon C. R, Lehman D.C and Manuselis G (2011). Textbook of Diagnostic Microbiology. Fourth edition. Saunders Publishers, USA.

Patrick R. Murray, Ellen Jo Baron, James H. Jorgensen, Marie Louise Landry, Michael A. Pfaller (2007). Manual of Clinical Microbiology, 9th ed.: American Society for Microbiology.

Wilson B. A, Salyers A.A, Whitt D.D and Winkler M.E (2011). Bacterial Pathogenesis: A molecular Approach. Third edition. American Society of Microbiology Press, USA.

Woods GL and Washington JA (1995). The Clinician and the Microbiology Laboratory. Mandell GL, Bennett JE, Dolin R (eds): Principles and Practice of Infectious Diseases. 4th ed. Churchill Livingstone, New York.

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