Introduction

Viruses are among the smallest biological entities on Earth. They are much smaller than bacteria and cannot be seen with a normal light microscope. Unlike living organisms, viruses cannot grow, obtain energy, or reproduce on their own. Instead, they must enter the cells of another organism and use the cell's machinery to make new viruses.

Viruses are responsible for many diseases in humans, animals, and plants, including the common cold, influenza, measles, and COVID-19. At the same time, viruses are important tools in scientific research and modern medicine. Understanding how viruses work helps scientists develop vaccines, antiviral medicines, and strategies to prevent the spread of disease.

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What Are Viruses?

A virus is a microscopic infectious particle that can reproduce only inside a living host cell.

Unlike bacteria, viruses are not made of cells.

Many scientists do not consider viruses to be living organisms because they:

• Cannot reproduce independently.
• Do not carry out metabolism.
• Do not grow.
• Cannot respond to their environment in the same way as living cells.

Outside a host cell, viruses are inactive.

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Figure 1. Viruses are tiny infectious particles made of genetic material enclosed in a protein coat.

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The Structure of a Virus

Although viruses vary in shape, most contain the same basic parts.

Genetic Material

A virus contains either:

• DNA, or
• RNA

This genetic material carries the instructions for making new viruses.

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Protein Coat (Capsid)

The genetic material is surrounded by a protective protein coat, called a capsid.

The capsid:

• Protects the genetic material.
• Helps the virus attach to host cells.

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Envelope (Some Viruses)

Some viruses have an outer lipid envelope taken from the membrane of the host cell.

The envelope contains proteins that help the virus recognise and enter new host cells.

Not all viruses have an envelope.

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Attachment Proteins

Many viruses have specialised surface proteins or spikes that bind to specific receptors on host cells.

These determine which organisms and which types of cells a virus can infect.

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Figure 2. Many viruses consist of genetic material inside a protein coat, and some also have an outer envelope with attachment proteins.

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How Do Viruses Reproduce?

Viruses cannot reproduce by themselves.

Instead, they infect a host cell.

The basic steps are:

Step 1 – Attachment

The virus attaches to a suitable host cell.

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Step 2 – Entry

The virus enters the cell or injects its genetic material.

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Step 3 – Replication

The viral genetic material takes control of the host cell.

The host cell begins making:

• Viral DNA or RNA.
• Viral proteins.

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Step 4 – Assembly

New virus particles are assembled inside the host cell.

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Step 5 – Release

The new viruses leave the host cell.

Some viruses cause the cell to burst, while others leave by budding through the cell membrane.

The released viruses can then infect more cells.

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Figure 3. Viruses reproduce by entering host cells and using the cell's machinery to produce new virus particles.

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Viruses vs Living Cells

Viruses are very different from living cells.

These differences are why viruses are generally not considered living organisms.

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Examples of Viral Diseases

Viruses cause many diseases in humans.

Examples include:

• Common cold.
• Influenza (flu).
• COVID-19.
• Measles.
• Chickenpox.
• Hepatitis B.
• HIV/AIDS.
• Polio.

Viruses also infect:

• Animals.
• Plants.
• Bacteria (these viruses are called bacteriophages).

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Figure 4. Viruses can infect humans, animals, plants, and even bacteria.

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Why Are Viruses Difficult to Treat?

Viruses are difficult to treat for several reasons.

They Live Inside Cells

Because viruses reproduce inside living cells, medicines must destroy the virus without harming the host cells.

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Antibiotics Do Not Work

Antibiotics kill bacteria by targeting structures and processes found in bacterial cells.

Viruses do not have these structures.

Therefore:

• Antibiotics are ineffective against viral infections.

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Viruses Can Change

Some viruses mutate rapidly.

These mutations can:

• Produce new virus strains.
• Reduce the effectiveness of treatments.
• Make vaccine development more challenging.

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Vaccines and Antiviral Medicines

Many viral diseases can be prevented through vaccination.

Vaccines help the immune system recognise viruses before infection occurs.

Some viral infections can also be treated using antiviral medicines, which slow or stop viral replication.

Unlike antibiotics, antiviral medicines usually do not kill viruses directly.

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Figure 5. Vaccines help the immune system recognise viruses and reduce the risk of disease.

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Helpful Uses of Viruses

Although many viruses cause disease, scientists also use viruses in medicine and research.

Examples include:

• Delivering genes during gene therapy.
• Producing some vaccines.
• Studying genetics.
• Treating certain bacterial infections using bacteriophages.

These applications show that viruses can also be valuable scientific tools.

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Why Viruses Matter

Viruses have major effects on:

• Human health.
• Agriculture.
• Wildlife.
• Evolution.
• Biotechnology.

Understanding viruses helps scientists develop better medicines, vaccines, and public health strategies to control infectious diseases.

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Worked Example

Question

State whether each statement is True or False.

1. Viruses are made of cells.
2. Viruses reproduce independently.
3. Antibiotics can cure viral infections.
4. Viruses contain genetic material.

Solution

1. False
2. False
3. False
4. True

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Real-World Connection

Every year, millions of people receive influenza vaccines to reduce their risk of catching the flu. Vaccines train the immune system to recognise viruses before infection occurs, allowing the body to respond more quickly if it is exposed. This reduces the chance of severe illness and helps protect vulnerable members of the community.

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Did You Know?

Some viruses are so small that thousands could fit across the width of a single human hair. Because they are much smaller than bacteria, scientists need powerful electron microscopes to see their detailed structure.

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Key Terms

Antiviral medicine – A drug that slows or stops the replication of viruses.

Capsid – The protective protein coat surrounding a virus's genetic material.

Envelope – An outer lipid membrane found around some viruses.

Host cell – A living cell that a virus infects and uses to reproduce.

RNA (Ribonucleic acid) – A nucleic acid that serves as the genetic material in some viruses and plays important roles in protein synthesis in living cells.

Virus – A microscopic infectious particle made of genetic material enclosed in a protein coat that can reproduce only inside a living host cell.

Vaccine – A preparation that stimulates the immune system to protect against a particular disease.

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Key Takeaways

• Viruses are microscopic infectious particles that reproduce only inside living host cells.
• A virus consists of genetic material (DNA or RNA) enclosed in a protein coat (capsid), and some also have a lipid envelope.
• Viruses reproduce by attaching to host cells, entering them, using the cell's machinery to make new viruses, assembling new virus particles, and releasing them.
• Unlike living cells, viruses are not cellular, do not carry out metabolism, and cannot reproduce independently.
• Viral diseases include influenza, COVID-19, measles, chickenpox, hepatitis B, and the common cold.
• Antibiotics do not work against viruses because viruses lack the cellular structures targeted by these medicines. Vaccines and antiviral drugs are the main tools used to prevent and treat viral infections.