Infection 4: How viruses cause disease Flashcards
Define the R0 or Basic Reproduction number (1)
this is the number of new infections one person will generate during their infectious period.
Describe the structure of a virus particle (3)
Nucleic acid (genetic material) DNA or RNA ds or ss \+ve/-ve/ambisense Protein coat Enveloped/unenveloped
What problems must a virus solve to infect a person
It must know what cell(s) to enter & how to enter the cell(s)
It must know how to replicate inside a cell
It must know how to exit & move from one infected cell to a new cell as well as to a new host in order to persist in nature
It must develop mechanisms to evade host defences
Outline the journey of how a virus causes infection
Encounter: virus meets host
Entry: virus enters host
Multiplication: virus replicates in the host
Spread: virus spreads from the site of entry
Damage: the virus, host response or both cause tissue damage
Outcome: the virus or the host wins, or they coexist
Name 4 ways a virus can enter the body
Skin: Abrasions Insect/animal bites Needle punctures Gastroenteritis viruses Movement facilitate viral entry Hostile environment Extreme acidity/alkalinity Digestive enzymes Mucus membranes; low pH Abrasions facilitate viral entry HPV - local lesions HIV – viral spread Localised infection – conjunctivitis Viral spread – eye blindness / CNS
Give 4 ways a virus can enter the blood for haematogenous spread
Ways of viral entry to blood: Directly through capillaries By replicating in endothelial cells Through vector bite By lymphatic capillaries Once in the blood, virus has access to almost every tissue
How is viraemia diagnosed? (1)
Diagnostic value – measuring viral replication
Define the terms neurotropic, neuroinvasive and neuvirulent
Neurotropic - virus can infect neural cells
Neuroinvasive – virus can enter CNS following infection of a peripheral site
Neurovirulent – virus can cause disease of nervous tissue
What are the 3 main mechanisms for how viruses injure cells?
Cytolytic viruses Inhibition of host protein and RNA synthesis – leads to loss of membrane integrity Syncytium formation Induction of apoptosis Non-cytolytic viruses CD8+ mediated CD4+ mediated B cell mediated Cell injury associated with free radicals
Explain the mechanism of latent infection and give an example
DNA viruses or retroviruses
Persistence of viral DNA
Extra-chromosomal element (herpes viruses)
Or integrated within the host genome (retroviruses)
During cell growth the viral genome is replicated along with the host cell chromosomes
Herpes simplex virus reactivation: fever, blisters or cold sores
Retrovirus infection may result in transformation of the cell leading to cancer
Give an example of a virus and how it evades the immune system
flu - antigen variatiom
EBV - IL10
Describe the process of antigenic shift and drift
shift - two strains join together
drift - mutation over time as they accumulate
