lecture recording on 18 February 2025 at 17.51.10 PM

Definition and Nature of Viruses

• Viruses are not considered living things because they do not meet criteria that define life (e.g., cells, metabolism, reproduction).

• When people say "killing a virus," a more accurate term is "destroying" it.

• Viruses are believed to have evolved from living organisms and possess a singular purpose: infection and replication.

Historical Context

• The 1918 flu pandemic was one of the first major pandemics caused by a virus, often mistaken for bacterial infections due to the historical lack of knowledge about viruses.

• It was not until the late 20th century that the influenza virus was isolated.

Structure of Viruses

• Viruses consist primarily of two components:

  • Nucleic Acid: Can be either DNA or RNA, but not both. It can be single or double-stranded.

  • Capsid: A protein coat that encases the nucleic acid.

  • Some viruses possess a viral envelope derived from the host’s cell membrane, obtained when the virus exits the host cell.

Classification of Viruses

• Viruses are classified based on:

  • The type of nucleic acid (DNA or RNA).

  • The structure of the capsid (shape and symmetry).

• Examples include:

  • Bacteriophages (viruses that infect bacteria), which have shapes resembling mechanical structures or geometrical figures.

Host Range of Viruses

• Host range refers to the specific species or types of cells a virus can infect, like:

  • Tobacco mosaic virus: Infects tobacco leaves.

  • Common cold viruses: Target the respiratory tract.

• Viruses are usually specific to certain host tissues and species, exemplified by rabies affecting specific animals.

Viral Replication Process

• Viruses replicate by hijacking the host cell’s machinery:

  • They enter a host cell, inject/introduce their nucleic acid, and take over cellular functions to produce more virus particles.

  • They proliferate by using the host's ribosomes, ATP, and Golgi apparatus to create viral components (capsids and nucleic acids).

  • This replication cycle can occur in two main ways:

    • Lytic Cycle: Where the virus actively replicates, eventually bursting the host cell and releasing new viruses.

    • Lysogenic Cycle: The viral DNA is integrated into the host DNA and can replicate unnoticed until triggered.

Impact of Viruses on Human Health

• Viral infections can range from mild to severe, with different viruses having varying transmission methods.

• Vaccination can prevent certain viruses (e.g., mRNA vaccines for COVID-19 help prevent the virus from attaching to cells).

• Certain viruses can remain dormant within a host for extended periods, later activating under specific triggers (e.g., HIV).

HIV and Other Notable Viruses

• HIV: Transmitted mostly through body fluids (sexual contact, sharing needles) and remains in many individuals without showing symptoms for years.

• Ebola: Known for causing severe hemorrhagic fever, but typically self-limiting; spread less effectively compared to HIV.

• Discussion on the ethics of conception when one has a transmissible condition like HIV includes considerations on genetic disorders and the implications for offspring.