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Infectious DiseasesGert-Jan Wiggers
,LE 1 – Introduction course Infectious Diseases
Infectious diseases
Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or
fungi; the diseases can be spread, directly or indirectly, from one person to another. Zoonotic diseases
are infectious diseases of animals that can cause disease when transmitted to humans.
What does an infectious agent need to do to infect the host?
1. Attachment entry into body: evade natural protective and cleansing mechanisms → entry
(infection)
2. Local or general spread in the body: evade immediate local defences → spread
3. Multiplication: increase numbers (many will die in the hose, or en route to new hosts) →
multiplication
4. Evasion of host defences: evade immune and other defences long enough for the full cycle in
the host to be completed → avoid killing by host defences
5. Shedding from body (exit): leave body at a site on a scale that ensures spread to fresh hosts
→ transmission
6. Cause damage in host: not strictly necessary but often occurs → pathology, disease
Common transmission routes
Respiratory or salivary spread, facial-oral spread and sexual spread.
• Skin: cold sore – herpes simplex virus; lesions – varicella zoster virus; warts – papillomavirus
• Milk: HVI, HTLV (both retroviruses)
• Blood: HIV, Hepatitis B and C virus
• Placenta: Rubella virus, cytomegalovirus, Zika virus
Horizontal vs Vertical transmission
• Horizontal transmission: person on person
• Vertical transmission: infection of next
generation (birth)
• Zoonoses: transmission from animals
- vector (biting arthropod
- vertebrate reservoir
- vector-vertebrate reservoir
What is a virus?
Obligate intracellular parasites or symbionts that possess their own genomes encoding information
required for virus reproduction and, thence, a degree of autonomy from the host genetic system, but
do not encode a complete translation system or a complete membrane apparatus.
Important terms
• Virion: (physical) virus particle
• Capsid: protein shell surrounding viral nucleic acids
• Nucleocapsid: capsid + nucleic acid
• Envelope: lipid membrane that surrounds the capsid (note that not all viruses are enveloped)
,General scheme virus replication cycle
1. Attachment to receptor on cell surface
2. Entry
3. Virus uncoating: virus enter cytoplasm
4. Replication
5. Assembly: capsids form around nucleic acid
6. Release: budding forms envelope
Key concept in virus classification
All viruses must make mRNA that can be read by host ribosomes.
Baltimore classification:
Baltimore classification is important to recognize to which class a virus belongs to and to know more
or less which steps are involved in the replication cycle. In addition you can think of a method to
tackle the virus.
Virus taxonomy
• Realm
• Order
• Family
• Genus
• Species
• Virus
Viruses within family / genus are not transmitted the same way, nor do they cause the same
diseases. Conversely, viruses form different families may cause similar symptoms.
, Detection of viruses
1. Diagnosis for patients with suspected virus infection
2. Fundamental virology/immunology research
3. Epidemiological research
4. Surveillance
What are the qualities of a good (diagnostic) test?
• Sensitive: few false positive
• Specific: few false negative
• Fast
• Cheap
• Easy to perform
Direct vs indirect laboratory diagnostics
Direct: detection of…
• Whole organism (infectious particle)
• Their structural components
- viral proteins
- genetic material
• Their products
- mostly relevant for bacteria/fungi
Indirect: detection of…
• Antibodies targeting microorganisms in serum of a patient
- IgG/IgA
Direct measures
• Detect (infectious virus particle
- culture based techniques
- (electron) microscopy
• Detect viral antigen (e.g., envelope protein)
- antigen ELISA
- fluorescence microscopy
- lateral flow assay
• Detect genetic material
- PCR
- sequencing
How small are viruses?
Detectable with light microscope and electron microscope.
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