02_2025S_MICR490_Intro to viruses

Definition of Viruses

• Obligate parasites needing host cell machinery for reproduction.

• Spread through infectious particles (virions).

• Microscopic agents with RNA or DNA genomes surrounded by protective coats.

Mechanism of Viral Action

• Entry into a host cell leads to the viral genome directing the synthesis of viral components through host cell systems.

• Progeny virus particles assemble within the infected cell.

Viral Presence

• Viruses' ubiquity in nature: 10^{15} viruses associated with the human body; 10^{30} in oceans, and significant excretion rates by infected animals.

Viruses in Living Organisms

• Viruses can infect a variety of hosts including humans, animals, plants, etc.

• They can cause significant economic and social impacts by affecting agriculture.

Human Diseases Caused by Viruses

• Examples include COVID-19, smallpox, yellow fever, and more significant global health concerns.

• Viruses account for approximately 15% of the cancer burden in humans.

Benefits of Viruses

• Viruses play crucial roles in biogeochemical cycles and maintaining microbiome homeostasis.

• Certain viral infections could mitigate the pathogenesis of other virus infections or bacterial pathogens.

Endogenous Retroviral DNA

• Approximately 8% of the human genome is derived from endogenous retroviruses, influencing evolutionary biology.

• Retroviral proteins contribute to essential reproductive processes like cell fusion in placentation.

Cross-Species Infection

• Viruses may cross species barriers, leading to zoonotic diseases.

• The rise in human populations and expansion into wilderness increases the frequency of such infections.

Historical Context of Viral Infections

• Co-evolution of viruses and humans leading to virulent strains over 10,000 years.

• Evidence of viral diseases documented in historical texts (e.g., The Iliad and Egyptian hieroglyphs).

The Evolution of Vaccination

• Variolation as a precursor to vaccination, with Cowpox leading to the discovery and term 'vaccination' by Edward Jenner.

• Importance of variolation in protecting against smallpox but with unpredictable outcomes.

Microbiology and Pathogenicity

• Antony van Leeuwenhoek's advancements in microscopy allowed the observation of microorganisms.

• Pasteur's experiments debunking spontaneous generation and establishing links between specific microorganisms and diseases.

Koch’s Postulates

• Criteria established by Robert Koch to link specific microorganisms to particular diseases, foundational to microbiology.

Discovery of Viruses

• Identification of virus-like infectious agents smaller than bacteria, beginning with Dimitrii Ivanovsky’s work on TMV.

• Beijerinck’s contributions in classifying these agents and naming them as viruses.

Defining Properties of Viruses

• Viruses are obligate intracellular parasites requiring host cells for propagation.

• Simplicity of virus structures identified via electron microscopy.

Viral Propagation Mechanisms

• Compartments containing viral genomes assembled in the host, relying on host biosynthetic machinery.

• Distinction between viral assembly and cellular reproduction.

Classification of Viruses

• Viruses classified based on nucleic acid type, symmetry of the capsid, presence of an envelope, and other characteristics.

• Baltimore classification based on mRNA production steps from diverse viral genomes.

Common Strategies for Viral Propagation

• Viral cycles include attachment, genome entry, replication, and particle assembly/release, reflecting shared strategies across virus types.