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Sources of infection and modes of infection transmission

Sources of infection.
Modes of transmission of infection.


Introduction :

 Man was and is still susceptible to different challenges in his surrounding environment, some of which were successfully defeated, but others are still there causing lots of problems.  A living example is infection or infectious diseases which constitute a minor problem in the developed countries, but a major one in the developing countries.  The outcome of such problem varies form nothing to a life threatening event.  An infection is an invasion of the body tissue by microorganisms and their proliferation within the tissue.  If the microorganism produces no clinical evidence of disease, the infection is called asymptomatic or subclinical.  Some asymptomatic conditions can cause severe damage to the host (1).  Microorganisms vary in their ability to produce disease. This ability is called virulence.  Microorganisms also vary in the source from where they emerge,  the severity of the disease they produce and their degree of communicability.  If the infectious agent can be transmitted to an individual by direct or indirect contact through a vector or vehicle, or as an airborne infection, the resulting condition is called a communicable disease.  In general, five groups of microorganisms can cause disease and these are: bacteria, viruses, fungi, protozoa, and rickettsia (2).  It is of importance to mention that the human body contains several types of microorganisms as a part of its normal architecture and these are called the “Normal Flora”.  In the following paragraphs, the sources of infection and the ways by which different infections are transmitted are going to be discussed.  In addition, examples of infectious agent will be presented for each  source of infection and mode of transmission.

Sources of infection:

 The sources of infection are numerous, and for each type of  infection a specific source becomes more significant than the others in the delivery of the infectious agent to the host, which is man in our discussion.  The sources of infection can be divided into two main groups.  These are exogenous and endogenous sources (3).  A source of infection is endogenous when the infectious agent comes from the patient’s own body, usually from his own normal flora. Endogenous sources of infections become important when the person’s own immunity against his normal flora becomes compromised such as in cases of contamination during surgery, malnutrition, impairment of blood supply and debilitating diseases such as AIDS, diabetes or any other accompanying infection.  Examples are the genera of staphylococci and streptococci which are normally found in the body, but can become pathogenic in certain circumstances (2).  The exogenous sources of infection introduce organisms from any where outside to inside the body, which is the case most of the time.  In addition to being exogenous, most of the time infections are transmitted from person to person or from animal to man (4). To be more specific, exogenous sources of infections can be either human, animal, or environmental in origin.  Humans can be a source of infection in three cases, either when they are clinically infected (symptomatic infection), when they are asymptomatically infected or when they are carriers (5).  In the first possibility, it is not uncommon that the causal organism can be shed from the host in large numbers leading to wider spread of the infection, but fortunately, it is easy to control the spread of such infections by treatment or by isolation.  An example is N. meningtidis which causes meningitis (4).    The problem is that when the infection is hidden and  the host’s circulation in the community continues to take place as in the case of being asymptomaticlly infected or being a carrier of the organism without knowing so as in the case of the carriers of HIV virus.  Humans can be a source of organisms which cause diseases that are sexually transmitted such as Treponema  pallidum which causes syphilis and N. gonorrhoae which causes gonorrheal infections or through blood when vectors act as vehicles as in the case of transmission of  Borrelia that causes relapsing fever (6).  Animals are another source of infection, and an infection derived from this source is called zoonotic infection.  Such infections are usually maintained in animals, and are acquired accidentally.  An example of such infections could be brucellosis caused by brucella mainly from cows and their products such as milk , rabies caused by rabies virus from wild animals, and plague which is caused by Pasteurella pestis.  Moreover, animal products such as meat, milk and eggs can be sources of infection.  Examples are E. Salmonella species and E. Cambylobacter (4).  Environmental sources are numerous and few environmental saprophytes are pathogenic for man unless in cases of individuals with severely compromised immune system.  But still some parasites may result in complications if introduced into the body from the environment.  Examples are some sporing bacilli of the genera Bacillus and Clostridium.  Food is another important and very common source of infection due to the everyday pattern of dealing with such material.  Food can be contaminated and hence a source of infection at several stages. At its origin (infected animal or plant), or at the time of processing when handled with hands or contaminated tools (2).  It is not only a vehicle when transmission is considered, but it is also a good environment where bacteria or any other pathogen can multiply and produce toxins (3). Water, the commonest and the most important material in life is a major source of infection only in case of being in contact with sewage. Examples of organisms derived from the previous two  sources when contaminated with faeces are Salmonella typhi associated with food poisoning and Shigella associated with Bacillary dysentery, Brucella causing undulant fever, Leptospira and Clostridium causing botulism. Examples of viruses are Poliomyelitis and Hepatitis A viruses.  Protozoa derived from contaminated food and water are Entamoeba histolytica which causes dysentery and Giardia lamblia which causes enteritis.  Examples of worms that are food or water borne are hookworms, pinworms and Ascaris.  Faecal bacteria such as E. coli can produce water borne outbreaks in case of water contamination with sewage (3).  Moreover, Soil, air and dust usually contain non pathogenic organisms of numerous types, but this is not always the case since pathogenic organisms can be introduced through them to humans causing diseases.  Soil can be contaminated with human or animal feaces that contain several pathogenic organism which have sporing capabilities enabling them to survive in harsh environments such as Clostridium tetani which causes tetanus, Cl. botulinum which causes food poisoning and B. anthracis which causes anthrax in animals mainly.  Examples of pathogenic fungi found in soil are Coccidioides immitis and Blastomyces (1).  Air can be contaminated with organisms shed from skin or the respiratory tract such as S. pneumoniae which causes pneumonia and S. pyogens which causes Scarlet fever, Corynebacterium diphtheriae which causes diphtheria, Haemophilus Influenzae, meningococci, anthrax bacilli  and Measles and Mumps viruses.  Infection with such organisms occurs by inhalation or ingestion of pathogen-containing droplets produced either orally or by the respiratory tract.  Dust doesn’t differ so much from the previous two sources in that it can be infected with organisms especially bacteria and viruses  that are shed from humans or any other source, and can be deposited in dust particles that can be all the time floating in the atmosphere such as Chickenpox virus.  Fomites are another source of infection which can be defined as any porous substance that can absorb and pass on contagion and almost all the previously mentioned organisms can be derived through this source (4).  Nosocomial infections are infections that were not present or were incubating at the time of admittance to a health care facility.  They include infections patients acquire during their stay in a healthcare facility or infections that may manifest after discharge.  These infections are considered more difficult to prevent and treat, more unpredictable, and more resistant to cure than infections contracted in the community.    Patients who undergo surgical procedures have a higher incidence of nosocomial infections than others.  The source of microorganisms that cause nosocomial infections can be the patients themselves, the healthcare facility, or the healthcare personnel (5).



Modes of transmission of infections:

 As previously mentioned, microorganisms that can infect man are every where in the air; on dust particles, food, and plants, on and in animals and humans; in soil and water; and on virtually every other surface.  Transmission of the microorganisms can be grouped into four main routes: contact, vehicle, airborne, and vectorborne.  The most frequent means of transmission is through contact with an infected host.  There are three types of contact transmission: direct contact as in cases of physical contact with an infected person especially in cases of local skin contact.  Diseases transmitted by this route are numerous and an example could be impetigo caused by Staphylococci. Another type of contact is the indirect one as in cases of contact with a contaminated object such as soil which may result in the transmission of organisms such as S. tetani , and droplet contact such as the contact with contaminated secretions from an infected person in saliva or milk as in the transmission of the virus responsible for Herpes cold sores (2).  Vehicle transmission occurs through a transporting agent or medium, such as food, water, or blood as in the case of the transmission of Hepatitis B virus , E. Salmonella or E. coli.  Microorganisms that remain on droplets or dust particles are carried through the air through airborne transmission (1).  The infectious agent is expelled from the carrier or symptomatically infected host in respiratory droplets during speaking, sneezing and coughing.  These are inhaled by another host.  In addition, droplets may adhere to dust particles that become a new source of infection when get into contact with tissues of a new host.  Examples of pathogens transmitted by such method are meningococci, staphylococci, Cornybacterium diphtheriae and measles virus (6).  A vector is an animal that transfers microorganisms from a reservoir to a host.  A vector picks up disease organisms from a source of infection, such as blood or feces, carries the infection within or on its body, and later deposits them where they infect a new host, directly or indirectly.  Mosquitoes, fleas, lice, flies, and ticks are common vectors of disease to humans such as malaria or leishmaniasis (5).  Sexual contact constitute another important route of transmission when precautions are not taken as in the classical example of HIV virus transmission,  in addition to N. gonorrhoae and Hepatitis B infection (1).  The faeco-oral route is important as a route of infection when hygienic standards are lacking.  Bowel excretions that contain the infectious microorganism can be either ingested directly through infected fingers or toilets or indirectly via food and water.  E. salmonellae, S. typhi,  viral hepatitis A and E and  V. cholerae are few examples of the huge number of organisms that can be transmitted this way (6).   Moreover, vertical transmission of infections from mother to her child either prenatally through the placenta, perinatally during passage of baby through the birth canal, or postnatally through breast feeding, is another important mode of transmission.  An example is the transmission of T. pallidum that causes syphilis (4).   It is worth mentioning that infectious diseases may be transmitted by one way or another or by more than one route.


 In conclusion, infections can be derived from several sources and transmitted by several means, and as usual prevention is better than cure.  The process of preventing infections can be accomplished by several means.  One is the prevention through basic cleansing and hygiene procedures, such as keeping the hands clean, washing and covering cuts and grazes, having wounds attended to by a physician, and seeking regular dental treatment.  The body itself resists infection by producing substances called antibodies, which act against infectious organisms.  The process by which resistance to infection is artificially produced is called immunization.  Moreover, safe sex, safe sanitary systems and safe animal keeping will probably always put man on the safe side.


1. Grist, Norman R., et. al., Diseases of infection.  2nd ed.  Oxford: Oxford University Press, 1993, P: 2- 11.

2. Duerden, B. I., et. al.  A new short text book of Microbial and Parasitic infection.  London:  Edward Arnold, 1987, P: 32 - 35.

3. Fuerst, Robert.  Frobisher & Fuerst’s MICROBIOLOGY in Health and Disease.  15th ed. Philadelphia:   W. B. Saunders Company, 1983, P: 354- 357.

4. Ellner, Paul D., and Neu, Harold C.   Understanding infectious diseases.  St. Louis:  Mosby Year Book, 1992, P:  5 - 9.

p>5. Mandal, Bibhat, et. al.  Lecture notes on infectious diseases.  5th ed.  London:  Black Well Sciences,  1995, P:  34.

6. Burton, Gwendolyn R. W.  Microbiology for the Health Sciences.  4th ed.  Philadelphia:  J. B. Lippincott Company, 1992, P: 203 -207.