Exercises: Viral Replication, R Value and Disease Emergence

Name the main parts of a typical virus particle and explain why viruses are described as obligate intracellular parasites.

Distinguish between a DNA virus and an RNA virus. Name one human disease caused by each.

Define the R value (reproduction number) of an infectious disease. State what an R value of 1 means and what an R value below 1 means for an outbreak.

Describe the stages of replication of a virus in a host cell, from attachment to release of new virus particles. State which host structures supply the materials and energy.

In an outbreak of influenza in a school in Galway, each infected person passes the virus to 3 others, and each case is infectious for one generation only. The first generation is 2 cases.

1. a) State the R value.

1. b) Calculate the number of new cases in generations 2, 3 and 4.

1. c) Calculate how many cases would occur in generation 4 if vaccination and isolation reduced R to 0.8, starting again from 2 cases.

Explain why RNA viruses such as influenza generally mutate faster than DNA viruses, and explain how this affects the need for a new influenza vaccine each year.

In 2020, a novel coronavirus spread rapidly across Ireland. The initial R value was about 3.0. Public health measures reduced it to 0.7.

1. a) Calculate the approximate number of new cases from 100 cases after four generations at each R value.

1. b) Explain two measures that could lower R and the biological reason each works.

1. c) Explain how herd immunity relates to R and calculate the minimum proportion of the population to be immune using $1-\frac{1}{R_0}$ for $R_0 = 3.0$.

HIV is a retrovirus with an RNA genome.

1. a) Describe how a retrovirus replicates, naming the enzyme involved.

1. b) Explain why HIV can remain dormant in the host genome.

1. c) Explain why the immune system is weakened and why patients then die from opportunistic infections.

1. d) Explain how antiretroviral drugs can target the replication cycle.

Many new human infectious diseases are zoonotic, passing from animals to humans.

1. a) Define emerging disease and explain what is meant by zoonosis.

1. b) Discuss four factors that contribute to the emergence of new diseases, such as deforestation, global travel, intensive farming and climate change.

1. c) Suggest how surveillance and a "One Health" approach (treating human, animal and environmental health as linked and managing them together) could reduce the risk in Ireland.