“ microbes that are normally found on or in the
human body without causing disease. Some of these
microbes, like intestinal residents, are necessary for
normal human function. ”
But still they may harm human body if local defense
mechanism is compromised or if they reach the areas
they should not be.
For example, E-coli reach the part of body other than
intestine may cause infection and ultimately disease.
◦ a microorganism capable of causing disease
◦ is the ability to cause disease
◦ degree of pathogenicity
◦ is the steps or mechanisms involved in the development of the disease
◦ is the study of the structural and functional manifestations of the
◦ Evidence of disease that is experienced or perceived
◦ Subjective changes in body function noted by patient but not
apparent to an observer
◦ Objective evidence of a disease the physician can observe and
◦ A specific group of signs and symptoms that accompany a
◦ An infection is the invasion and multiplication of microbes in
an individual or population.
◦ Is a disease caused by a microbe
◦ Disease is when the infection causes damage to the
individual’s vital functions and systems.
◦ Disease is a wider term covers genetic and other diseases
too, but it is often used besides Infectious disease.
1. Source/reservoir of infection
2. Mode of transmission
3. Portal of entry
4. Portal of exit
Once these pathogens are inside your body they cause
disease because they reproduce rapidly.
Bacteria do this by splitting in two.
The often produce toxin at the same time.
Viruses actually take over cells, damaging and
◦ A supply of pathogen/infection/disease.
◦ For example, female anopheles mosquito is a reservoir of
Plasmodium falciparum for malaria.
◦ Likely or liable to be influenced or harmed by a pathogen.
Reservoir and Susceptible host
both are equally dangerous
Microbes enter in body by various
1) Contact Transmission
a) Direct Contact Transmission
Person to person transmission. No intermediate object is involved.
b) Indirect Contact Transmission
Agent is transferred via a nonliving object.
Examples: towels, eating
utensils, thermometers, stethoscopes, bedding, clothes, money, and
c) Droplet Transmission
Microbes are spread in mucus droplets that travel short distance (less than
Examples: sneezing, coughing, talking and laughing.
2) Vehicle Transmission:
Transmission of disease via medium such as
water, food, air, blood, body fluids, and intravenous fluids.
a) Waterborne Transmission
Usually caused by water contaminated with sewage.
b) Airborne Transmission
Spread of agents by droplets in dust that travel more than 1m
3) Vector Transmission
Animals that carry disease from one host to
another, Arthropods (insects) are most important animal
a) Mechanical Transmission
Passive transport of pathogens on insect’s body. For
b) Biological Transmission
Pathogen spends part of its life cycle in the vector. For
Site at which microbes leave body. Most common exit
portals are respiratory and gastrointestinal tracts.
a) Respiratory Tract:
◦ Exit in discharges (mucus droplets) from nose and mouth.
Transmission by coughing, sneezing, spitting, etc.
◦ Examples: Tuberculosis, influenza etc.
b) Gastrointestinal Tract:
◦ Exit in feces or sliva.
◦ Feces: Cholera, typhoid fever, salmonella.
◦ Saliva: Rabies virus, herpes simplex 1.
c) Urogenital Tract
◦ Exit in secretions of urine
d) Skin and Wound infections
◦ Spread through direct contact or through fomites.
◦ Transmission through insects, needles, and syringes.
Insects: Malaria, yellow fever etc.
Needles: AIDS and Hepatitis B.
Gain entry to the host.
Colonise the host’s tissues.
Resist the host’s defences
Damage the host’s tissues.
◦ Provides defense against infection by a number of
inherent, mechanical, and chemical barriers
Nonspecific defense mechanism
g, perspiring, and
Skin, mucous, mem
branes, and other
Internal Defense Mechanism
Hygiene refers to conditions and practices that help to maintain health
and prevent the spread of diseases. Medical hygiene therefore includes a
specific set of practices associated with this preservation of health, for
example environmental cleaning, sterilization of equipment, hand
hygiene, water and sanitation and safe disposal of medical waste.
Sanitation generally refers to the provision of facilities and services for the
safe disposal of human urine and faeces. Inadequate sanitation is a major
cause of disease world-wide and improving sanitation is known to have a
significant beneficial impact on health both in households and across
communities. The word 'sanitation' also refers to the maintenance of
hygienic conditions, through services such as garbage collection and