Virus-Host Interactions (Lecture 23)
• Viruses propagate locally in the host at or near the site of entry into susceptible
cells. Spread to distant sites via blood stream or lymphatics.
• Virulence – the capacity of a virus, compared to other closely related viruses, to
produce disease in a host
• Neurovirulence – the ability of a virus to cause disease in the brain or nervous
• Even the most virulent viruses can cause subclinical, inapparent infections that
outnumber the cases of symptomatic illnesses (handled by innate or specific
o Innate immune responses – non-specific
o Adaptive immune responses – (specific) anti-viral antibodies and cytolytic
Determinants of tissue and cell susceptibility
• Each virus has a characteristic host range evident at 2 levels:
o The degree of species specificity
o Cell or tissue specificity the major determinant of the pattern of disease
observed in the individual organism. Can be determined at several stages
of the virus multiplication cycle.
Diverse life cycles characterized in cell culture comprise patterns of infection
• Acute infections – virus produced 7-14 after infection, symptoms appear and virus
is cleared in a month.
o Influenza virus, rhinovirus (common cold), and mumps virus
o Clearance of infection is due to vigorous immune responses to the virus as
well as lack of persistent or latent infections caused by these viruses.
• Persistent infections – virus infection is not cleared either due to poor immune
response, lack of access of the immune response to the infectious site (such as in
the brain), or rapid changes in the virus so that the virus evades the host immune
o Virus and viral proteins continually produced throughout life of host.
o Symptoms may or may not appea