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Infectious Disease
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Infectious Disease
 An infectious disease is a transmissible disease
resulting from the presence of pathogens including
prions, viruses, bacteria, fungi, protozoa and
multicellular parasites.
 These pathogens are able to cause disease in animals
and/or plants.
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Pathogen Transmission
 Transmission can be
 Direct contact – directly from person to person
 Fluid transfer (HIV)
 Pathogen cannot survive outside of host
 Vector borne
 Malaria – Anopheles mosquitoes
 Water borne
 Cholera
 MattDamon Bourne
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Some Definitions
 Transmission cycle is the way in which a pathogen
passes from one host to another.
 Control methods attempt to break the transmission
cycle
 Major control methods
 Vaccination
 Barriers
 Sterilization of instruments
 Disinfecting or antiseptic procedures
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Some Definitions (Page 2)
 Incidence – Number of NEW cases in a time period
 Prevalence – TOTAL number of infected individuals
 Endemic – When the disease is always found in a
population
 Prevalence may be high or low
 Epidemic – Sudden or dramatic increase in the
incidence of a disease in an area or country
 Pandemic – Worldwide epidemic
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WHO (World Health Org.) Data
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Comparison by Income
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Pathogen Class & Disease
Bacterial
Viral
TB
(Tuberculosis)
HIV / AIDS
Protist
Malaria
Cholera
Smallpox
Measles
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Cholera (Overview)
 Caused by the bacterium Vibrio cholerae
 3% death rate
 Water-borne
 Infection occurs by ingestion of fecal
contaminated fluids or food
 Profuse diarrhea and vomiting
 Due to enterotoxin production
 (3 to 5 US gal) of diarrhea a day.
 High bacteria load in diarrhea (rice water)
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Cholera (Overview 2)
 Endemic in Africa and areas without access to
 If treated, recovery is quick
 If untreated, mortality > 50%
 Treatment
 Oral rehydration therapy (ORT)
 In severe cases
 Potassium replenishment due to acidosis
 Antibiotic therapy
 Doxycycline (Vibramycin)
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Cholera: Pathology
 The bacteria lodge in the small intestine & multiply
 Produce an enterotoxin called choleragen
 Disrupts epithelium functions by inhibiting a G-Protein
 This leads to excess salt exiting the epithelium into the
intestinal lumen
 Now the intestinal lumen is hypertonic, so
 Excess water is secreted into the intestinal lumen from
the epithelial linings
 AND the patient loses copious amounts of water via
diarrhoea
 Can be fatal if untreated for 24 hours
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Cholera: Cause & Transmission
 Vibrio Cholerae (bacterium) is
ingested
 Fecal contaminated of water or
food
 Usually unclean water supply
 Especially in poor nations
 Sometimes oysters or shellfish
 If living in waters infected with
human sewage
 Mainly found in wealthy nations
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Cholera is a top 10 cause of death, WHY?
 Very rare in the US
 Nearly all domestic cases (0-5 per year) are attributed to
exposure abroad
 Common in recently industrialized countries
 Africa has the highest case fatality rate
 Large population migrations into urban centers in developing
countries strain existing water and sanitation infrastructure
and increasing disease risk.
 Cholera epidemics are a marker for poverty and lack of basic
sanitation.
 An ongoing global pandemic in Asia, Africa and Latin
America for the last four decades.
 2010 Haiti outbreak following earthquake = 303 deaths
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Cholera is a social issue
 No sewage treatment or clean water in growing
populations of developing countries
 No financial resources to tackle large municipal projects
such as providing drainage system and clean water
supply.
 Inadequate cooking or washing in contaminated water.
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Measles
 Highly contagious respiratory disease
 Measles virus = pathogen AND measles = disease
 Death Rate = 1-2 /1,000
 7-10 day course of infection
 Rash throughout body and flu-like symptoms

Children may spike high fevers
 Characterized by frequent complications & highly
contagious nature
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Measles (Page 2)
 Transmitted via breathing, coughing or
sneezing.
 CDC: “It is so contagious that any child
who is exposed to it and is not immune
will probably get the disease.”
 Easily prevented by a common
childhood vaccine that is recommended
for all children
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Measles Prevalence
 Prior to vaccine availability, almost every child
in the US (and parts of Europe) contracted
measles.
 Now, only sporadic cases remain in the US and
other Americas.
 But deaths do occur in India and Asia
 2010 Worldwide: 20 mil. cases AND 164,000
deaths
 Anti-vaccine movement
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Malaria
 Malaria is caused by the protoctist
(protist) Plasmodium
 It is an Anopheles mosquito-borne
infectious disease
 Anopheles is considered a vector
(transmits the pathogen)
 Confined to tropical areas
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Mosquito Vector
 The female Anopheles mosquitoes feed on
human blood to obtain protein
 Plasmodium may be present in Anopheles’
saliva, so a bitten by an infected Anopheles,
may take up some of the plasmodium.
 The female is only sucking blood, so how
does the plasmodium get injected into the
human?
 Once in the blood, the plasmodium
parasites travel to the liver to mature and
reproduce.
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Mosquito Vector (Page 2)
 The infective stages of the
Plasmodium enter the red blood
cells, where they multiply.
 When the infected mosquito feeds
again, it injects an anti-coagulant
into the blood meal so that it flows
out from the host into the
mosquito.
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Clarification…
 How does a vector differ from a pathogen?
 Can you get infected from a mosquito bite, when the
mosquito does not harbor the plasmodium parasite?
 Can you get infected from a male mosquito bite, when
the mosquito has the plasmodium parasite?
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Prevention of Malaria
 The three main ways to control
malaria:
1. Reduce the number of mosquitoes
(destruction of breeding ground,
stagnant water, the use of
insecticides)
2. Avoid mosquitoes bites (using fly
screens, mosquito repellents)
3. Use prophylactic drugs to prevent
infection (chloroquine and quinine,
proguanil, mefloquine).
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Current Issues in Malaria Treatment
 An increase in drug-resistance forms of Plasmodium.
 An increase in proportion of cases caused by
Plasmodium falciparum (more lethal form)
 Difficulties in developing a vaccine

An increase in the number of epidemics because of
climatic and environmental changes that favours the
spread of mosquitoes (Global Warming is bad?)

the migration of people as a result of civil unrest and
war.
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HIV / AIDS
 AIDS = Acquired Immune Deficiency
Syndrome (NOT Disease)

Due to a collection of opportunistic infections
 Caused by the retrovirus HIV (Human
Immunodeficiency Virus)
 HIV Disease is caused by HIV pathogen
 HIV infects Helper T-cells

CD4 cells
 Involved in which acquired immunity?
 So how is it pathological?
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-- 2 copies of ssRNA = viral genome
-- Has reverse transcriptase (Why?)
-- Proteases for exiting cells
-- Glycoprotein 120 and 41 in envelope
-- p24 in the conical capsid
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HIV / AIDS Pathology
 HIV infection results in the gradual destruction of the
immune system by destroying the CD4 cells and
macrophages
 Since the immune system is compromised, patients are
susceptible to Opportunistic Infections (OIs)
 Patients cannot defend against OIs, and die ultimately
of the OI, but really died due to the loss of immune
system functionality

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HIV Disease
 Infection is characterized by a brief flu-like disease
 Called Acute HIV syndrome
 2-4 weeks following exposure
 Clinical Latency
 Patient will be asymptomatic for multiple years
 3-20 years after exposure
 Progressive appearance of symptoms
 People may not know they are HIV carriers
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AIDS
 Following the progressive degeneration of the immune
system, eventually the patient runs out of T-cells
 Diagnosis: T-cell (CD4) count < 200
 Coincides with the appearance of an OI
 End stage of HIV disease
 PCP – pneumocystis Carnii pneumonia (40%)
 Esophageal Candidiasis, wasting disease
 Viral induced cancers (Kaposi’s sarcoma)
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Transmission
 Unprotected sexual intercourse

Primarily anal (0.03-3.00% risk) and vaginal (0.05-0.30%)

Risk varies dramatically between high risk (low income
countries) and lower risk (high income countries)
 Exposure to bodily fluids or tissues (blood to blood)
 Birth (25% risk)
 Risk can be reduced to 1% with Anti-Retroviral Therapy
 Intravenous drug usage (0.67%)
 Inappropriate syringe usage in sub-Saharan Africa
 Blood transfusions (90%)
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Social Issues
 HIV infection weakens the immune system thus people
are more vulnerable to malnutrition, TB and malaria.
 AIDS cause an adverse effect on the economic
development of countries
 Purchase of expensive drugs drains government funds
 AIDS has reversed 10-15 years of economic growth for
some African states.
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Treatment
 No cure and no vaccine for HIV
 Drug therapy can slow down the onset of AIDS
 The drugs have side effects and are expensive.
 Two or more drugs (combination therapy) can prevent
the replication of the virus inside the host cells.

Delays progression of the disease
 May lead to atherosclerosis and cardiomegalopathy
 The pattern and timing of medication through the day
must be strictly followed.
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Global Distribution of HIV
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Tuberculosis
 Tubercle Bacillus (TB)
 Tubercle is a round nodule or wart-like structure
 Mycobacterium = pathogen
 Airborne
 An invasive disease - it starts with a primary infection in
the lungs and quickly spreads to the lymph nodes,
bones and gut.
 It often strikes HIV-positive people when their immune
system begins to weaken.
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Characteristics
 Most commonly affects the lungs (pulmonary TB)
but can also affect the CNS, lymphatic system, and
even skin.
 Some people become infected and develop TB
quite quickly, whilst in others the bacteria remain
inactive for many years.
 People with this inactive infection do not spread
the disease to others.
 But the bacteria become active when these
people are weaken by other diseases, suffer from
malnutrition or become infected with HIV.
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Transmission Method
 Generally, it is spread via
airborne droplets, and is
particularly prevalent in
overcrowded areas, and people
suffering malnutrition are more
susceptible.
 TB is spread when infected
people with the active form of
the illness cough or sneeze
 The bacteria are carried in the
air in tiny droplets of liquid.
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TB due to Bovine?
 M. Bovis also causes tuberculosis
and occurs in cattle, it can be
spread via their meat and milk, but
both bacterium infections have
fallen now because of vaccine
introduction in the 1950s.
 People can also contract TB from
cattle through airborne droplets
 Pasteurization has made this
virtually irrelevant
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Treatment
 Diagnosis - sputum sample is searched by microscope
for identification of the tuberculosis bacteria
 Treatment always involves patient isolation
 Treatment involves multiple antibiotics to ensure that
all bacteria are killed, especially the drug resistant
strain of the bacterium.
 Long course of treatment (9 months to a year)
 Incomplete treatment course may lead to infecting
others with drug-resistant forms (evolved) of TB.
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TB Resistance
 Tuberculosis is unfortunately showing a comeback, and
this is thought to be due to a variety of factors.
 These include;
 Breakdown in the vaccination and control program
 Poor housing causing overcrowding
 The AIDS epidemic weakening immune systems and
allowing it to be more prevalent
 Some strains are now resistant to antibiotics
 Multiple Drug Resistant (MDR-TB) &
Extremely Drug Resistant (XDR-TB)
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TB Prevalence
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TB Mortality
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Current Issues
 Frontline “TB Silent Killer” in Swaziland
 NDM-1 plasmid gene
 NDM-1 was first detected in a Klebsiella pneumoniae
isolate in 2008. It was later detected in India, Pakistan,
the UK, US, Canada, and Japan
 Escherichia coli and Klebsiella pneumoniae, but the gene
for NDM-1 can spread from one strain of bacteria to
another by horizontal gene transfer.
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