Viral Replication, R Value and Disease Emergence

Structure of a virus and why it needs a host

A virus is not a cell. It has no cytoplasm, no ribosomes and no metabolism of its own. It is a particle: genetic material in a protein coat. Because it cannot make proteins or release energy by itself, it can only reproduce inside a living host cell. We say viruses are obligate intracellular parasites.

A typical virus particle has:

• Genetic material: either DNA or RNA (never both), single or double stranded.

• Capsid: a protein coat built from repeating units that protects the genome.

• Envelope (in some viruses, e.g. influenza and HIV): an outer layer taken from the host cell membrane, carrying viral proteins.

• Surface proteins (antigens): used to attach to the host cell. These are what antibodies recognise.

DNA viruses and RNA viruses

The type of nucleic acid is used to classify viruses.

Why do RNA viruses mutate faster? Copying RNA uses enzymes (RNA polymerases) that have little or no proofreading, so copying errors stay in the new genomes. DNA replication in the host uses polymerases that check and repair mistakes. More errors mean more mutations, so more variation in the surface proteins.

Viral replication in a host cell

Most viruses follow the same sequence, sometimes called the lytic cycle.

1. Attachment: surface proteins bind to specific receptors on the host cell. This is why a virus infects only certain cell types.

2. Entry: the virus (or just its genome) gets into the cell, by endocytosis or by fusing with the membrane.

3. Uncoating: the capsid breaks down and releases the genome.

4. Replication and synthesis: the host's enzymes, nucleotides and ribosomes copy the viral genome and make viral proteins. The host supplies the raw materials (amino acids, nucleotides), the ribosomes and the ATP (from the mitochondria).

5. Assembly: new genomes and capsid proteins join to form new virus particles.

6. Release: particles leave by bursting (lysing) the cell, or by budding off and taking some membrane as an envelope.

Retroviruses and HIV

A retrovirus has an RNA genome but makes a DNA copy of it. It carries the enzyme reverse transcriptase, which uses viral RNA as a template to build DNA. HIV is the example you must know.

1. HIV attaches to receptors on helper T cells and enters.

2. Reverse transcriptase makes DNA from the viral RNA.

3. The DNA is inserted into the host cell's chromosomes (by the enzyme integrase). It is now called a provirus.

4. The host cell transcribes and translates the provirus, making new viral RNA and proteins.

5. New particles assemble and bud from the cell.

Because the provirus is part of the host genome, it can stay dormant for years. The cell copies it every time it divides, and the virus is hidden from the immune system because few viral proteins are made.

In time HIV destroys helper T cells. These cells coordinate both B cell and killer T cell responses, so the immune system is weakened (AIDS). The patient then dies not from HIV itself but from opportunistic infections (such as pneumonia or certain cancers) that a healthy immune system would normally clear.

Antiretroviral drugs target the cycle: some block reverse transcriptase, some block integrase, and some stop the virus entering the cell or assembling. Using a combination makes it harder for the virus to evolve resistance.

The R value and the spread of an outbreak

The R value (reproduction number, $R_0$ at the start of an outbreak when nobody is immune) is the average number of new people each infected person goes on to infect.

• $R > 1$: each case causes more than one new case. The outbreak grows.

• $R = 1$: each case replaces itself. The number of cases stays steady.

• $R < 1$: each case causes fewer than one new case. The outbreak dies out.

If cases in one generation are $C$, then the next generation has $C \times R$ new cases. After $n$ generations: $C \times R^{n}$.

Herd immunity. If a fraction of the population is immune, each case meets fewer susceptible people and R falls. To bring R below 1 the immune proportion must be at least

For $R_0 = 3.0$: $1 - \tfrac{1}{3} = 0.67$, so about 67% must be immune. Measles has a much higher $R_0$ (about 15), so over 90% must be immune. This is why the childhood immunisation schedule matters.

Lowering R. Measures work by reducing contact or the chance of transmission: isolating cases (fewer contacts), hand washing and masks (fewer virus particles reaching a new host), ventilation, vaccination (more immune people, so fewer susceptible hosts).

Emerging diseases

An emerging disease is one that has newly appeared in a population, or one that has existed but is rapidly increasing in incidence or spreading to a new area. A zoonosis is a disease that passes from animals to humans (e.g. COVID-19 and avian influenza are thought to have crossed from animals).

Factors that help new diseases emerge:

• Mutation: especially in RNA viruses, a change may allow a virus to infect a new species or evade immunity.

• Persistence in a host: a virus that hides in a host (such as HIV, or an animal reservoir) can survive and spread later.

• Antibiotic resistance: bacteria resistant to antibiotics (such as MRSA) are harder to treat and spread in hospitals.

• Global travel and mobility: an infected person can cross the world in hours, so a local outbreak becomes a pandemic.

• Land use change and deforestation: brings people closer to wildlife reservoirs.

• Intensive farming: large numbers of closely packed animals allow rapid spread and mixing of viruses.

• Climate change: moves the range of insects like mosquitoes and ticks that carry disease.

• Low population immunity: nobody is immune to a novel virus, so $R_0$ is high.

Surveillance and One Health. The One Health approach recognises that human, animal and environmental health are linked. In Ireland, the HSE and Health Protection Surveillance Centre monitor human cases, while the Department of Agriculture and Teagasc monitor livestock and wildlife (for example avian influenza in wild birds). Sharing data, testing wastewater, controlling antibiotic use on farms and vaccinating early can detect a new threat before it spreads.

Back to the start: viruses spread fast because each case can cause several new ones ($R > 1$) and they use our own cells to copy themselves, and we need a new flu vaccine because RNA viruses mutate quickly.

Summary

• A virus is a particle of DNA or RNA in a protein capsid (sometimes with an envelope). It is an obligate intracellular parasite and replicates only inside a host cell.

• Replication: attachment, entry, uncoating, copying and protein synthesis (using host ribosomes, enzymes and ATP), assembly, release.

• RNA viruses mutate faster than DNA viruses because RNA copying is poorly proofread, which is why the flu vaccine changes every year.

• Retroviruses such as HIV use reverse transcriptase to make DNA, which is integrated into the host genome and can lie dormant. Loss of helper T cells leads to opportunistic infections.

• $R$ is the average number of new cases per case: above 1 the outbreak grows, at 1 it is steady, below 1 it dies out. Herd immunity needs at least $1 - \tfrac{1}{R_0}$ of the population immune.

• Emerging diseases arise through mutation, zoonosis, travel, farming, deforestation, climate change and antibiotic resistance. Surveillance and One Health reduce the risk.