
Infectious Diseases

Definitions
disease
: deviation in the normal structure or function
of body organ or system
•Infection
: Entry, establishment & multiplication of a
microorganism in the body
•Infectious disease:
disease caused by infectious agent
•Symptoms
: features of a disease that is felt by the patient.
•Signs
: features or manifestations of a disease that is
observed by the doctor.
•Clinical features
: symptoms & signs of a disease

Incubation period
: time interval between onset of infection
& appearance of symptoms.
host
: organism that harbor & nourish another organism
(parasite)
Carrier
: A person or animal with asymptomatic infection that
can be transmitted to another person or animal.
Pathogen
: A microorganism capable of causing disease.
Pathogenicity
: ability of infectious agent to cause disease

Toxigenicity
: ability of a microorganism to produce a toxin.
Opportunistic pathogen
: microorganism that cause a disease
only when the host's resistance is impaired.
Virulence
: quantitative ability of an agent to cause disease.
Virulence involves invasiveness & toxigenicity
Invasiveness
: process by which microorganisms enter &
spread in the body

Mode of transmission of infectious agents
• I
ngestion: e.g., Typhoid
• I
nhalation: e.g., infleunza
• I
noculation: e.g., malaria
Infectious agents
(Prions, Viruses, Chlamydia, Mycoplasma, Bacteria,
Fungi, Protozoa, Helminthes)

Prions
•An infectious protein particle without nucleic acid.
•Obligate intracellular agents
•Resistant to common disinfectant e.g., ethanol,
, but susceptible to household bleach & iodine
•Causes kuru, Crutzfeldt-Jacob & bovine spongiform
encephalitis

Viruses
• Are obligate intracellular agents
• Contain DNA or RNA within a cylindrical or spherical
protein coat (capsid), which may be surrounded by
lipid envelope.
• Viral infection can cause:
1. Acute illness (e.g., common cold, infleunza etc)
2. Chronic disease (e.g., AIDS)
3. Latent infection with occasional re-activation
e.g., Herpse viruses

Bacteria
•Simple cell (prokaryotic: Karyo- = nucleus) w/o true nucleus
i.e., chromosomes not surrounded by nuclear membrane
•Divided into 2 main groups according to Gram stain reaction
(Gram positive bacteria looks purple-violet or Gram negative
deep pink)
•Extracellular like streptococci or intracellular like
mycobacterium tuberculosis

Chlamydia, Rickettsiae and
Mycoplasmas
Are similar to bacteria but lack certain structures (a cell wall-
mycoplasma) or metabolic capabilities (ATP synthesis-
chlamydia)
Chlamydia cause genitourinary infections, conjunctivitis, and
or respiratory infections of newborns.
Rickettsia are transmitted by insect vectors, including lice,
tick, and mites and causes hemorrhagic vasculitis,
pneumonia, hepatitis and encephalitis.
Mycoplasma bind to the surface of epithelial cells and cause
atypical pneumonia or urethritis

Fungi
•Eukaryotic (has true nucleus, mitochondria like
human cells)
•Cause superficial infection (e.g., candida) or deep
infection in immune compromized persons as in
AIDS patients

Protozoa
•Single celled eukaryotic organisms.
•Trichomonas vaginalis transmitted sexually
•Entamoeba histolytica & Giardia lamblia transmitted
by ingestion
•Plasmodium & leishmania transmitted by insect

Helminthes
- Are highly differentiated multicellular
organisms with complex life cycles.
-
Cause disease in proportion to the
number of infecting organisms
-
Roundworms, hookworms, flatworms.

Tissue response to infectious agent
1.Suppurative inflammation
2.Necrotizing inflammation
3.Granulomatous inflammation
4.Interstitial inflammation
5.cytopathic-cytoproliferative response

Suppurative inflammation:
Acute inflammatory response
with neutrophil infiltration, & purulant exudate.
Typically seen in pyogenic bacterial infections
e.g., Staphylococci & streptococci (bacterial
meningitis, Bacterial pneumonia)

Necrotizing inflammation
Virulent organism producing sever tissue damage
and extensive cell death e.g.(Necrotizing
fasciitis , Necrotizing pharyngitis)

Granulomatous inflammation
* Granulomatous response
predominates
* Slow- growing organisms
- Mycobacteria
-Fungi
- Parasites

Interstitial inflammation
- Diffuse mononuclear interstitial infiltrate
- As in interstitial lung disease can caused by
long term exposure to hazardous materials
such as asbestos.
And autoimmune disease as rheumatoid
arthritis or even viral infection as myocarditis
, hepatitis and others.

Cytopathic-cytoproliferative response
Structural changes in host cells that are caused
by viral invasion.
Characterized by virus induced damage to
individual
host cells with little or no inflammatory
response
May show inclusion bodies (intranuclear or
cytoplasmic
aggregate of viral components or blisters
(e.g., herpse viruses) or apoptosis (as in viral
hepatitis)

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