Virus Virulence and R0 Value

Study Notes

R0 (reproduction number) measures how contagious an infectious disease is, representing the number of new cases generated by a single infected person in a completely vulnerable population.
R0 only applies when everyone in the population is completely vulnerable, meaning no one has been vaccinated, had the disease before, or has a way to control the spread of the disease.

Virulence is the capacity of a virus to cause disease in a host, ranging from mild to lethal effects.
Virulence can be quantitated by measuring:
- Mean time of death
- Mean time to appearance of symptoms
- Fever, weight loss, and pathological lesions
- Reduction in blood CD4 cells (e.g., HIV)
Virulence is influenced by factors such as dose, route of infection, species, age, gender, and host susceptibility.
Virulence genes are usually identified by mutation, where a virus causes reduced or no disease in a specified system.

Tropism refers to the spectrum of tissues infected by a virus, ranging from limited to pantropic.
Determinants of tropism include susceptibility, permissivity, accessibility, and defense.
Examples of tissue tropism:
- Human influenza virus: infects respiratory tract
- Avian H5N1 virus: infects many cells other than respiratory tract
- SARS CoV-2: not restricted to respiratory tract, with TMPRSS2 and furin cleavage site

Viruses can cause immunosuppression, globally reducing the immune response during infection.
Mechanisms of immunosuppression include:
- Replication in immune system cells (e.g., HIV)
- Disturbing cytokine production and intracellular signaling
- Viral proteins acting as viroceptors or virokines (immune modulators)

Mechanisms of cell injury by viruses:
- Cytolytic viruses: cytopathic effects (apoptosis, necrosis, pyroptosis)
- Viroporins
- Viral inhibition of host proteins and RNA synthesis, leading to loss of membrane integrity and cytoplasmic degradation
- Syncytium formation by enveloped viruses (e.g., parainfluenza, HIV)

Age: very young and very old individuals are most susceptible to disease
Gender: males are slightly more susceptible to viral infections than females
Pregnancy: hep A, B, E, influenza are more lethal, and polio is more common
Malnutrition increases susceptibility due to compromised physical barriers and immune response
Cigarette smoking and stress (e.g., HSV) can also affect virulence