Ch. 13

Chapter 13: Viruses and Prions

Introduction to Viruses

Viruses are microscopic infectious agents that play a significant role in human health, agriculture, and global disease outbreaks. Understanding viruses is crucial for microbiology students due to their impact on society and their unique biological properties.

• Viruses are responsible for many diseases, including COVID-19, avian flu, and measles.

• They are a major focus of public health due to their potential for rapid spread and severe outbreaks.

• Advances in molecular biology have enabled the discovery and characterization of new viruses.

Example: The COVID-19 pandemic (caused by SARS-CoV-2) and the ongoing threat of avian influenza highlight the importance of viral research.

Recent Advances and Emerging Viruses

Modern molecular techniques have led to the identification of several new and emerging viruses in recent decades.

• 1983: Discovery of Human Immunodeficiency Virus (HIV)

• 2003: Identification of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV)

• 2020: Emergence of COVID-19 (SARS-CoV-2)

• The World Health Organization (WHO) lists viruses as the top emerging pathogens likely to cause severe outbreaks in the near future.

What Are Viruses?

Basic Properties of Viruses

Viruses are unique biological entities that differ fundamentally from cellular life forms.

• They are acellular (not made of cells).

• They require a living host cell to replicate (obligate intracellular parasites).

• Viruses lack:

  • Ribosomes (cannot synthesize proteins independently)

  • Cell membranes (unless enveloped by host-derived membrane)

  • Ability to produce energy (no metabolic machinery)

• Viruses always contain nucleic acids (either DNA or RNA, never both).

Poll Question Example: Which of the following do viruses lack? (A) ribosomes, (B) cell membranes, (C) ability to produce energy, (D) nucleic acids, (E) A, B, and C. Correct answer: E (A, B, and C)

Why Do Viruses Matter?

Impact on Health and Society

Viruses are significant due to their ability to cause widespread disease, economic disruption, and influence on public health policy.

• Examples of important viruses: COVID-19, Avian Flu, Measles, HIV.

• Viruses can have high transmission rates (measured by R0, the basic reproduction number).

• Some viruses, like measles, are extremely contagious (R0 up to 18).

Additional info: R0 is a key epidemiological metric indicating how many people, on average, one infected person will transmit the virus to in a susceptible population.

Summary Table: Key Properties of Viruses

Property	Viruses	Bacteria	Eukaryotic Cells
Cellular Structure	No (acellular)	Yes (prokaryotic)	Yes (eukaryotic)
Genetic Material	DNA or RNA (never both)	DNA (usually)	DNA
Ribosomes	No	Yes	Yes
Metabolism	No	Yes	Yes
Replication	Only in host cell	Binary fission	Mitosis/meiosis

Conclusion

Viruses are essential to study in microbiology due to their unique biology, their role in disease, and their impact on society. Understanding their structure, replication, and epidemiology is foundational for careers in health sciences, research, and public health.