This document provides an overview of infectious diseases. It begins with definitions of key terms like infection, pathogens, and hosts. It then covers classifications of diseases, stages of infectious diseases, and epidemiological aspects. It discusses the transmission of diseases and important historical figures like Koch. It also outlines different types of pathogens, how diseases are produced, and virulence factors. The document concludes with sections on laboratory diagnosis and principles of antibacterial therapy.

1. Infectious
Diseases
Dr Adam Ibrahim
MBBS
Amoud University

2. Introduction of infectious disease
 Overview
 Classification of disease
 Stage of infectious disease
 Epidemiological aspects of infectious disease
(in a population and individual level)
 Pathogens
 Disease
 Laboratory diagnosis of infectious disease

3. Overview
Infection is the term used to indicate the
presence of an infectious agent in an
individual or population.
Infectious diseases are the invasion of a host
organism by microbes.
Microbes that cause illness are also known
as pathogens.

4. The most common pathogens are bacteria
and viruses; some kinds of fungi and
protozoa, also cause disease.
An organism that a microbe infects is
known as the host for that microbe.

5. Infectious diseases remain a leading cause
of death and disability.
Advances in the control of diseases such as
tuberculosis (TB), malaria, and smallpox
after the Second World War led to hopes
that the threat from infectious disease would
diminish.

6. Factors such as increasing international
travel and trade, the breakdown of public
health systems in some countries due to war
or economic decline and growing resistance
to existing antibiotics have meant that
infectious diseases continue to be a
significant risk to health

7. There has been ongoing progress in controlling
some vaccine-preventable childhood diseases such
as polio, diphtheria and invasive Haemophilus
influenza type b infection (Hib).
However, new previously unknown diseases have
appeared in the past 25 years such as acquired
immune deficiency syndrome (AIDS) and
hepatitis C.

8. Well-known diseases such as measles have
become more common again after a period of
decline.

9. Human activities that have led to the
emergence and spread of new diseases
 Encroachment on wildlife habitats. The
construction of new villages and housing
developments in rural areas brings people
into contact with animals--and the microbes
they harbor.
 Changes in agriculture.
 Destroying rain forests.

10.  Uncontrolled urbanization
 Modern transport.
 High-speed globe-trotting. With
international jet-airplane travel, people
infected with a new disease can carry the
disease to the far side of the world before
their first symptoms appear.

11. Koch’s Postulates
Koch developed four criteria to demon-
strate that a specific disease is caused by a particular
agent.
1. The specific agent must be associated with every
case of the disease.
2. The agent must be isolated from a diseased host and
grown in culture.
3. When the culture-grown agent is introduced into a
healthy susceptible host, the agent must cause the
same disease.
4. The same agent must again be isolated from the
infected experimental host.

12. Infectious vs. noninfectious disease
Any disease caused by a pathogen is an
infectious disease; any disease not caused
by a pathogen is a non-infectious disease
Not all infectious diseases are spread from
host to host.

13. Not all infectious diseases are associated
with pathogen infection of the host
(e.g., intoxications can result from exposure
to secreted toxin rather than the secreting
organism).

14. Classification Of Disease
Diseases may be classified in a number of
ways, many of which we will subsequently
discuss,
including:
– Infectious disease
– Communicable disease
– Noncommunicable disease
– Contagious disease

15. Communicable disease
A communicable disease is an infectious disease
that may be passed from individual to individual.
Yet, not all infectious diseases are considered
communicable
A more narrow definition of communicable
disease is one that is infectious during the
incubation or recovery (convalescence) periods of
a disease

16. Noncommunicable disease
A noncommunicable disease is an infectious disease
that is not spread from individual to individual but
instead is acquired from an inanimate object or other
species
Noncommunicable infectious disease may be
differentiated into three categories:
Infections caused by opportunists arising from an
individual’s own microflora
Poisonings following ingestion of secreted exotoxins
Infections acquired from organisms found in the
environment (other than from individuals of one's own
species

17. Contagious disease
A communicable diseases that is easily
passed from individual to individual is said
to be contagious
Yes, not all communicable disease are
considered contagious (though certainly all
contagious diseases are considered
communicable)

18. TYPES OF INFECTIOUS DISEASE
Infectious disease can be described as being
Acute, chronic, sub acute, latent, or as an
in apparent (subclinical) infection
Local, focal, or systemic
Septicemia, bacteremia, viremia, or
toxemia
A primary/ secondary infection, super
infection, or a mixed infection

19. Classification of Infectious Disease
By duration
– Acute – develops and runs its course quickly.
– Chronic – develops more slowly and is usually less severe, but may
persist for a long, indefinite period of time.
– Latent – characterized by periods of no symptoms between outbreaks
of illness.
By location
– Local – confined to a specific area of the body.
– Systemic – a generalized illness that infects most of the body with
pathogens distributed widely in tissues.
By timing
– Primary – initial infection in a previously healthy person.
– Secondary – infection that occurs in a person weakened by a primary
infection.

20. Phases of Infectious Disease
Diseases caused by infectious agents usually run a
standard course that is associated with different
signs and symptoms.
Before we describe these different phases of
infectious diseases, we must first define the
terms, “signs” and “symptoms.”

21. Signs of an infectious disease are characteristics of
a disease that can be observed by examining a
patient.
They include things such as
fever, coughing, rash, vomiting, and diarrhea.
Symptoms, on the other hand, can be felt only by
the patient. They include pain, headache, and
nausea.

22. STAGES OF INFECTIOUS DISEASE
Infectious diseases tend to occur in stages
including (in typical order)
Incubation period
Prodromal phase (not typical)
Invasive phase
Decline phase
Convalescence period
Sequelae (not typical)

23. Phases of Infectious Disease
Incubation period – time between infection
and the appearance of signs and symptoms.
Prodromal phase – mild, nonspecific
symptoms that signal onset of some diseases.
Clinical phase – a person experiences typical
signs and symptoms of disease.
Decline phase - subsidence of symptoms.
Recovery phase – symptoms have
disappeared, tissues heal, and the body regains
strength.

24. Infection in Individuals – definition
Depending upon the host-microbe
interaction, infection can be:
Silent
(inapparent, asymptomatic, subclinical), or
Overt, causing a disease of infection.
Silent or asymptomatic infections, such as
HIV infection during the early phase, can
still be infectious.

25. A carrier is a person who is infected with an
organism but shows no evidence of
disease, although disease may have been
present earlier
Acute infection implies a 'short-lived'
infection, such as influenza, with or without
symptoms; the period of infectivity is also
short.

26. Chronic infection refers to a 'long-standing'
condition during which the pathogen continually
replicates and the patient may be persistently
infectious to others, e.g., hepatitis B infection.
Latent infection refers to a persistent infection
with the possibility of intermittent shedding of
pathogens, e.g., varicella zoster virus causing
shingles or herpes simplex virus causing cold
sores or genital herpes

27. Infection in Populations
Endemic infection refers to infection or disease
that occurs regularly at low or moderate
frequency.
Pandemics are global epidemics. The size of
'outbreaks is dependant upon factors such as the
ratio of susceptible to immune subjects, period of
infectivity, population density, etc.

28. Spread of Infection
With respect to the spread of
infection, people can be divided in to :
Those who are susceptible,
Those who are infected but are not yet
infectious,
Those who are infected and infectious, and
Those who are immune.
Recovery from infection usually gives
immunity.

29. Means of transmission
Contact requires direct or indirect contact (
fomites, blood, or body fluids)
Food ingestion of contaminated food or water
water
Airborne inhalation of contaminated air
Vector- borne dependent on biology of vector as
well as infectivity of organism
Perinatal similar to contact infection, however, the
contact may occur in uterus or during the delivery

30. Transmission of Infectious
Diseases
Agents that cause infectious diseases can be
transmitted in many ways.
– Through the air
Courtesy of VOA
– Through contaminated food or water Chinese students
wearing masks during a
– Through body fluids SARS outbreak
– By direct contact with contaminated
objects
– By animal vectors such as
insects, birds, bats, etc.
Courtesy of CDC
Aedes aegypti mosquito
Known to transmit
Robin Cochran-Dirksen (Many slides Dengue fever
from BioEdOnline Baylor Christine
Herrmann PhD)

31. Incubation Period
The incubation period is defined as the time
between exposure to the onset of symptoms
or other signs of infection.
Diseases have incubation periods that vary
widely

32. Infectivity
Ability of agent to cause infection
Number of infectious particles required
In person-to-person transmission, secondary
attack rate is a measure of infectivity
In person-to-person transmission, the higher the
infectivity, the more secondary infections. For
example, the infectivity of tuberculosis is low and
the infectivity of smallpox is high.

33. Pathogenicity
Ability of a microbiological agent to induce
disease
Because the smallpox virus can produce
disease relatively easily, it is considered to
have high pathogenicity.
Polio has low pathogenicity.

34. Immunogenicity
Ability of an organism to produce an
immune response that provides protection
against reinfection with the same or similar
agent
Can be life long or for limited periods
Important information for development of
vaccines

35. Herd immunity
 When the number of immune persons is
high enough that it is unlikely that a
susceptible person will have contact with an
infected person
 Level of immunity required is dependent
on the characteristics of the infectious
disease

36. Pathogenic mechanism
Direct tissue invasion
Production of a toxin
Immunologic enhancement or allergic
reaction
Persistent or latent infection
Enhancement of host susceptibility to drugs
Immune suppression

37. Pathogen
The pathogens fall into five main groups:
1. Viruses
2. Bacteria
3. Fungi
4. Protozoa
5. Helminthes

38. Definitions
Commensalism: Literally `eating at the same
table'! This refers to a neutral situation where the
host and bacteria live together, but have no effect
on each other's life cycle - either positive or
negative..
Parasitism: This describes an unequal relationship
where one organism clearly benefits from an
association to the detriment of another.

39. Opportunistic infection: This occurs when the
normal human defences are so weakened that it
allows infection to take place by organisms that
would not generally be able to cause infection in a
healthy human.

40. Nosocomial infection: These are infections
that are transmitted in hospitals.
Some of these may be opportunistic
infections mentioned above affecting
seriously ill patients, others, for example
infections with Methicillin-Resistant
Staphylococcus aureus (MRSA), may occur
because of the special nature of the hospital
environment

41. Bacterial pathogenesis
Host susceptibility
Bacterial infectivity
Host resistance
Genetic and molecular basis for virulence
Host mediated pathogenesis
Intracellular growth
Virulence factors

42. Disease
Disease is a disturbance in the state of
health (of the host) wherein the body cannot
carry out all its normal functions
When an infection causes disease, the
effects of the disease range from mild to
severe
Disease, or illness, is characterized by
changes in the host that interfere with
normal function

43. Production of Disease
Once a bacterium has entered the
body, there are a number of ways in which
it can cause disease.
Local Effects
Distant Effects( Endotoxin and exotoxins)
Evasion of Host Defences

44. Virulence factors
Properties of a pathogens that allow them
to cause disease are termed virulence
factors
Virulence factors may distinguish a
pathogenic microorganism from otherwise
identical non-pathogenic microorganisms
by allowing pathogens to invade, adhere
to, and colonize a host, and then harm the
host

45. Types of virulence factors
The following are types of virulence
factors:
Adherence Factors: Many pathogenic bacteria
colonize mucosal sites by using pili (fimbriae)
to adhere to cells.
Invasion Factors: Surface components that
allow the bacterium to invade host cells can be
encoded on plasmids, but more often are on the
chromosome.

46. Capsules: Many bacteria are surrounded by
capsules that protect them from opsonization and
phagocytosis.
 Endotoxins: The lipopolysaccharide endotoxins on
Gram-negative bacteria cause fever, changes in
blood pressure, inflammation, lethal shock, and
many other toxic events.
Exotoxins: Exotoxins include several types of
protein toxins and enzymes produced and/or
secreted from pathogenic bacteria. Major
categories include cytotoxins, neurotoxins, and
enterotoxins.

47. Laboratory diagnosis of infectious
disease
Is based one or more of the following:
Direct examination of the specimen by
microscopic examination or antigenic
technique.
Isolation of organism in culture
Sero – diagnosis.
Molecular genetic detection.

48. Laboratory diagnosis of bacteria and
fungal infection
Microscopy : gram stain remain extremely
useful diagnostic technique because it is a
rapid and inexpensive method.
 Isolation and identification of organism:
 blood culture , CSF culture .
 urine/stool/genital/throat/respiratory culture.

49. Laboratory diagnosis of viruses
infection
 By electronic microscopy
 Antigenic detection
 Viral isolation in culture
 Serologic detection
 Detection of viral genome by molecular
biology technique

50. Laboratory diagnosis of parasitic
infection.
Most parasites are detected by
– Microscopic examination of clinical specimen:
– e.g. plasmodium, bobesai spp; leishmania spp;
helminthes egg, etc.
– Blood smear
– Fecal specimen.
Serological diagnosis are good for
trichinella, toxoplasmosis, and strongyloidiasis.

51. Principles of Antibacterial Therapy
age-appropriate antibiotic dosing and
toxicities must also be considered.
Specific antibiotic therapy based →
microbiologic diagnosis → supported by
antimicrobial susceptibility testing.
Common practice is based on a clinical
diagnosis with empirical use of
antibacterial agents.

52. Empiric use of antibacterial agents
Appropriate empirical use of antibacterial
agents in infants and children is essential.
A complete history + physical
examination+ appropriate laboratory +
radiographic studies= specific diagnoses=
empiric antibacterial use.
Antibiotic action is related to achieving
therapeutic levels at the site of infection.

53. The Patterns Of Antimicrobial
Resistance
Resistance to penicillin and cephalosporin
antibiotics.
Antimicrobial resistance occurs through
many modifications of the bacterial
genome

54. Thank you for your attention