An overview of the different sources and routes of transmission of infection

1. Source and Transmission of
Infection
Aman Ullah (Lecturer)
B.Sc. Med. Lab. Technology
PhD Research Scholar (Microbiology)
Certificate in Health Professional Education
Institute of Paramedical Sciences,
Khyber Medical University, Peshawar, Pakistan

2. Introduction
• Man was and is still susceptible to different challenges
in his surrounding environment
• 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
• Microorganisms vary in the source from where they
emerge

3. 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
• Exogenous and endogenous sources
• A source of infection is endogenous when the infectious agent comes from
the patient’s own body, usually from his own normal flora
• Examples are the genera of staphylococci and streptococci which are
normally found in the body, but can become pathogenic in certain
circumstances
• The exogenous sources of infection introduce organisms from any where
outside to inside the body
• 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

4. Sources of Infection
• 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 asymptomatically 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 or through blood when vectors act as
vehicles
• Animals are another source of infection, and an infection derived
from this source is called zoonotic infection
• Environmental sources are numerous and few environmental
saprophytes are pathogenic for man unless in cases of individuals
with severely compromised immune system

5. Sources of Infection
• Food is another important and very common source of infection due to
the everyday pattern of dealing with such material
• 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
• 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
• Air can be contaminated with organisms shed from skin or the respiratory
tract
• Infection with such organisms occurs by inhalation or ingestion of
pathogen-containing droplets produced either orally or by the respiratory
tract
• Fomites are another source of infection which can be defined as any
porous substance that can absorb and pass on contagion

6. Transmission of Infection
• 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
• 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
• 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

7. Transmission of Infection
• Microorganisms that remain on droplets or dust particles are carried
through the air through airborne transmission
• 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
• 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
• The faeco-oral route is important as a route of infection when hygienic
standards are lacking

8. Transmission of Infection
• 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
• 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
• It is worth mentioning that infectious diseases may be transmitted
by one way or another or by more than one route.

10. Questions/Suggestions
khurramthalwi@hotamail.com