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Infection and disease
Disease
 Disease can be thought of as any
condition that impairs the normal
functions of an organism.
 Diseases can be divided into two
broad categories:
 Non-infectious diseases
 Infectious diseases
Non-infectious diseases:
 Are not caused by an infectious
agent
 May be the result of more than one
factor: environmental, genetic and
biological factors may contribute to
their development
 Cannot be transmitted from one
person to another
Non-infectious diseases include:
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Nutritional (deficiency diseases)
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e.g. scurvy, anorexia
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Sickle cell disease, cystic fibrosis, Huntington disease
Congenital birth defects
Chromosomal disorders/genetic changes leading to cancers
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Smoking - Cancer
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Bipolar disorder, schizophrenia
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Alzeihmers disease, Parkinsons disease
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Cardiovascular disease
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Type I diabetes
Genetic
Environment (social diseases)
Mental diseases
Degenerative diseases
Diseases of physiological malfunction
Autoimmune diseases
Infectious diseases:
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The germ theory of disease, also called the pathogenic
theory of medicine, is a theory that proposes that microorganisms are the cause of many diseases.
Highly controversial when first proposed
Now a cornerstone of modern medicine and clinical
microbiology, leading to such important innovations such as
antibiotics and hygienic practices.
Louis Pasteur, Robert Koch and Joseph Lister are given
credit for it’s acceptance in the later half of the 19th century
(around the time Warrnambool was settled).
Germ theory took a long time to be accepted – surgeons
did not begin to wear masks in surgery until the 1890s.
Important Discoveries:
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Louis Pasteur (1860s)
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Joseph Lister (1860s)
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Showed microorganism could be found in air but were not generated by air
Demonstrated that there is no microbial contamination when air is withheld
Demonstrated that rotting of food is caused by bacteria
Observed that wounds left open to air were prone to infection
Concluded that bacteria which Pasteur indicated could rot food might also be
cause of infections
Starting using carbolic acid (which he knew was highly poisonous to living
organisms) on patients, his own hands and around hospital rooms
Incidence of infection in patients was dramatically decreased – first practice of
antiseptic surgery
Robert Koch (1870s)
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First person to demonstrate that relationship between a specific microorganism
and a specific disease – this lead to the development of Koch’s postulates
Ignaz Semmelweis:
The forgotten scientist
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Lister has been quoted as saying “Without Semmelweis my
work would be nothing” but who was Semmelweis???
Semmelweis was a Hungarian doctor teaching medicine in
Vienna in the 1840s
He observed that childbirth fever (peurperal fever) was
killing around 13% of new mothers in one ward of his
hospital which was staffed by doctors and students, while
only 2% of new mothers were dying of this same disease in
another ward staffed by nurses and midwifes. Why the
difference?
In 1947, Semmelweis’ friend Jakob Kolletscha was
performing an autopsy on a woman who died of childbirth
fever when he punctured his finger with a knife and died of
similar symptoms.
Ignaz Semmelweis:
The forgotten scientist cont…
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An autopsy of Kolletscha indicated the same pathological findings as
childbirth fever
Semmelweis proposed a relationship between the performing of
autopsies and the spread of childbirth fever
He immediately instigated a policy of using chlorinated lime for
washing hands between performing autopsies and examining patients,
and the death rate in the ward staffed by doctors and students
dropped to a level similar to that of the other ward
In 1848 he extended his policy to washing all instruments that were
coming in contact with women in labour and his records indicated that
childbirth fever was virtually eliminated from the hospital
A number of different factors limited the speed with which his ideas
caught on – when his ideas and results were finally published in 1861
they received much unfavourable criticism
Disgusted with the medical establishment, Semmelweis eventually had
a nervous breakdown and was admitted to a mental hospital where he
eventually died of infection.
Infectious and contagious
What’s the difference?
 Contagious diseases are infectious diseases
that are capable of spreading rapidly from
one person to another by contact or close
proximity.
Occurrence of infectious disease
Within a population, an infectious disease may be:
 Sporadic – occasional occurrence
 e.g. localised cases of chicken pox
 Endemic – regular, continuing occurrence
 e.g. malaria in parts of Africa
 Epidemic – significantly increased occurrence
 e.g. Horse influenza in NSW 2007
 Pandemic – epidemic occurrence in multiple countries
 e.g. the Spanish flu of 1918-1919
Infectious diseases are caused by
pathogens
 A pathogen is a parasite,
microorganism or other agent which
causes an infectious disease.
Infections caused by pathogens
 Infections caused by pathogens can be
divided into two types: primary and
opportunistic.
 Primary infections cause disease when
they infect a host, they are not usually
associated with the host e.g. plague
(Yersinia pestis) and influenza.
 Opportunistic infections cause disease in
some cases, but can be part of normal flora
at other times e.g. Pseudomonas, Candida
What is normal flora?
Bacteria found normally in or on the body
More bacterial than human cells in the body
Provide some nutrients (vitamin K)
Stimulate immune system, immunity can
be cross-reactive against certain pathogens
 Prevent colonization by potential
pathogens (antibiotic-associated colitis,
Clostridium difficile)
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How do pathogens cause disease?
 To understand how pathogens cause
disease we need to understand how it
infects a host and the effects it has
on that host.
Three steps in causing disease
1. Transmission of pathogen
2. Grow and reproduction of pathogen
3. Damage to host
Transmission
 There must be some way for a successful
pathogen to move from one host to the
next.
 Modes of transmission:
 Airborne
 Waterborne
 Passed directly from one host to the next by
direct contact or exchange of bodily fluids
 Present in food
 Carried by other living organisms (vectors)
Growth and/or Reproduction
 In order to cause disease pathogens
must be able to grow and/or
reproduce in the host.
Damage to the host
 Damage to the host is generally due to one
or more of the following factors:
 Invasion of tissue – breakdown of tissue and
cell death. Damage to cells from bacterial
enzymes
 Production of toxins
 Inhibition of normal cell function
 Depriving host of nutrients
 Production of damaging proteins (e.g. viruses
and prions)
 Body’s own reaction to presence of pathogen:
fever, inflammation and swelling.
Koch’s Postulates
 Proposed by Robert Koch (1884)as a way of proving
that microorganisms are the cause of a particular
disease.
1. Microorganism must be present in every case of the
disease
2. Organism must be grown in pure culture from disease
hosts
3. Produce the same disease from the pure culture
4. Organism recovered from experimentally infected
hosts
Molecular Postulates
 Describe virulence factors – more
applicable to new diseases e.g. SARS
1. Virulence gene or its product must be
present
2. Virulence gene must transform a nonpathogen into a pathogen
3. Virulence gene must be expressed during
disease process
4. Antibodies against gene products are
protective
Types of pathogens
 Pathogens may be cellular or non-cellular.
 Some parasites may be pathogenic, while
others do not cause diseases directly but
are important in transmitting diseases (e.g.
fleas transmitting the plague).
 Cellular pathogens or pathogenic
organisms include: bacteria, protozoa,
oomycetes, fungi, worms and arthropods.
 Non-cellular pathogens or pathogenic
agents include: viruses, viroids and prions.