A virus is a tiny, infectious agent that can only reproduce inside the living cells of an organism. Viruses are much smaller than bacteria and far simpler in structure. They lack the machinery needed to replicate on their own and rely on host cells to provide the necessary resources.

• Viruses come in various shapes and sizes, but all share some common features:
  • Genetic material: They contain either DNA or RNA as their genetic material, but never both.
  • Protein capsid: A protective protein shell surrounds the genetic material.
  • (Some viruses also have an) Envelope: A membranous layer derived from the host cell surrounds the capsid in some viruses.

What are the main features of a virus?

• Obligate parasites: Viruses cannot reproduce on their own. They invade host cells, hijack cellular machinery, and use it to produce new viral particles.
• Specificity: Different viruses have specific host ranges, meaning they can only infect particular types of cells or organisms.
• High mutation rate: Viral genomes can mutate rapidly, allowing them to evolve and evade host defenses.

Why is a virus important?

• Viruses are the most abundant biological entity on Earth and play a significant role in ecosystems.
• They cause a wide range of diseases in humans, animals, and plants, impacting global health and agriculture.
• Studying viruses is crucial for developing vaccines, antiviral drugs, and understanding fundamental biological processes.

What are the applications of a virus in practice?

• Vaccinations: Vaccines work by exposing the body to weakened or inactive forms of a virus, stimulating the immune system to develop immunity and prevent future infections.
• Gene Therapy: Viral vectors, modified viruses, can be used to deliver therapeutic genes into cells for treating genetic diseases.
• Phage Therapy: Certain viruses (bacteriophages) specifically target and kill bacteria. This approach is being explored as an alternative to antibiotics for combating antibiotic-resistant infections.

Practical example

• The common cold is a classic example of a viral disease. Rhinoviruses, belonging to a group of single-stranded RNA viruses, are the most frequent culprits. These viruses infect the upper respiratory tract, causing symptoms like runny nose, sore throat, and coughing.

Critical comments

• Viral infections can range from mild to life-threatening, and some viruses lack effective treatments.
• The rapid evolution of viruses poses a constant challenge in developing vaccines and antiviral drugs.
• The misuse of viral vectors in gene therapy raises biosafety concerns.