1, In-Depth Notes on Viruses

Definition: Viruses are obligate intracellular parasites that cannot replicate outside of their host cell.
Host range: Refers to the range of different organisms that can be infected by a virus. Each virus has a limited host range.
Tissue tropism: Within a host, a virus may only infect specific tissues (e.g., Rhabdovirus infects neurons).

Viruses carry the instructions for reproduction in their genome.
They hijack the host cell's machinery to produce new viruses, specifically controlling the cell's ribosomes for this purpose.
Virally infected cells often get damaged due to the infection.
Outside a host, viruses exist as metabolically inert particles called virions.

All viruses share a basic structure: a nucleic acid core surrounded by protein.
Types of nucleic acids: Can be either DNA or RNA; can be linear or circular, single-stranded or double-stranded.
RNA viruses can be segmented or non-segmented.

Capsid: Most viruses have a protein sheath called a capsid around their nucleic acid core.
Enzymes: Some viruses carry specialized enzymes necessary for their replication (e.g., reverse transcriptase).
Shapes of Viruses:
- Helical capsid: Can be rod-like or thread-like, length determined by nucleic acid.
- Icosahedral capsid: Composed of 20 small triangles, giving it nearly spherical symmetry.
- Complex viruses: E.g., T-even bacteriophages have a binal (twofold) symmetry with a helical tail and an icosahedral head.
- Enveloped viruses: Many animal viruses have an outer lipid bilayer derived from the host cell membrane.

Vary greatly in nucleic acid type and number of strands (most RNA viruses are single-stranded).
Common RNA viruses include those responsible for influenza and the common cold, which replicate in the host cell's cytoplasm.
Mutation rates in RNA viruses are high, posing challenges for vaccines and treatment.
Retroviruses: Single-stranded RNA viruses that use reverse transcriptase to create double-stranded DNA (e.g., HIV).
DNA viruses: Generally double-stranded, replicating in the nucleus of eukaryotic host cells (e.g., smallpox).

The Baltimore Classification system classifies viruses based on the relationship between genome structure and gene expression.
Replication mechanics: DNA viruses utilize host replication enzymes, while RNA viruses typically need their own.
- + Strand RNA: Same sequence as mRNA.
- - Strand RNA: Complementary to mRNA.
Both types rely on the host's tRNA and ribosomes for protein synthesis.

Persistent infections: May remain latent or chronic, causing long-term issues, whereas acute infections result in rapid virus replication and symptom onset.
Resolution characteristics:
- Acute diseases resolve completely (e.g., flu).
- Chronic diseases persist and may recur.
- Latent infections may not present symptoms initially but can reappear later.

Known for historical pandemics, including the 1918 influenza pandemic.
Structure: Enveloped, Type V animal virus with surface proteins Hemagglutinin (H) and Neuraminidase (N).
Antigenic drift: Small mutations in H and N proteins make prior vaccines ineffective, necessitating yearly flu shots.
Antigenic shift: Major genetic recombination leads to new strains, which can result in pandemics.

Infects respiratory epithelium via spike protein binding to ACE2.
Symptoms vary from asymptomatic to severe respiratory distress, pneumonia, or even death.
The infection-to-fatality ratio varies significantly by age demographic.

Prions: Infectious proteins leading to diseases such as BSE (mad cow disease) and Creutzfeldt–Jacob disease.
Prions cause normal proteins to misfold, which exacerbates the disease.
Recent discussions have been raised regarding a potential novel prion-like disease related to brain conditions in New Brunswick.