Viruses

Antigenic Shift
- Definition: A phenomenon where different viruses infect the same host cell and recombine their genetic material.
- Characteristics:
- Major changes occur in a relatively short time in the surface proteins of viruses.
- Can lead to the emergence of totally new viral strains that go unrecognized by the immune system, resulting in potential pandemics.
Antigenic Drift
- Definition: Small, incremental changes in the viral genetic material that accumulate over time.
- Characteristics:
- Gradually produces variation in the surface proteins of the virus.
- The immune system eventually fails to recognize these variants due to accumulated changes.
- Example:
  - Influenza vaccine: Requires adjustment each year to remain effective against changing strains.
  - HIV: Undergoes rapid antigenic drift even within a single individual, complicating immune control efforts.
- Mechanism: Due to high error rates in viral replication, especially in RNA viruses, there’s a necessity to convert RNA to DNA before replication occurs.

Structural Diversity in Viruses
- Viruses vary in shape and size but are classified as obligate intracellular parasites.
- Examples of structures in viruses:
- Bacteriophage (specifically Bacteriophage Lambda):
  - Components include:
  - Capsid
  - DNA
  - Neck
  - Tail sheath
  - Tail fibres
  - Glycoprotein spikes
  - Viral envelope
  - Reverse transcriptase for retroviruses.
- HIV (Human Immunodeficiency Virus):
- Type: Retrovirus
- Function: Converts RNA into DNA through reverse transcriptase, thereby infecting white blood cells (CD4) leading to AIDS (Acquired Immune Deficiency Syndrome).

Virus First Hypothesis:
- Suggests that viruses originated before cells, transitioning from simple to more complex entities.
Regressive Hypothesis (Reduction or Degeneracy):
- Proposes that viruses began as small cells that adapted to living within larger cells, eventually shedding unnecessary structures through gene reduction.
Escape Hypothesis (Vagrancy):
- Hypothesizes that viral genetic material (DNA or RNA) escaped from larger cells and became encapsulated by a protective boundary.

Epidemiology:
- The study focused on the occurrence, distribution, and control of diseases within populations.
Outbreak:
- An unexpected spike in cases of a specific health condition.
Epidemic:
- An outbreak that exhibits rapid growth over a larger geographical area.
Pandemic:
- A global epidemic characterized by exponential growth.
Not Considered Alive:
- Viruses do not perform life functions independently; they require a host cell for replication and metabolism.
Host Cell:
- Any cell that a virus utilizes to carry out its metabolic and reproductive processes. Different viruses target specific types of cells due to their structure and genetic material.
Virus Characteristics:
- Contain nucleic acids (either DNA or RNA, which can be double-stranded or single-stranded).
- Enclosed within a capsid and may possess an envelope for host cell attachment.
- Not considered living organisms as they lack cytoplasm and other cellular structures.

Examples of Viruses and Their Target Cells:
- HIV:
- Infects white blood cells.
- Hepatitis:
- Targets liver cells.
- Poliovirus:
- Infects nerve cells.
Virus Replication Process:
- Viruses utilize the host’s cellular machinery for protein synthesis and reproduction as they lack necessary components such as mitochondria or enzymes.

Two Main Reproductive Cycles:
- Lytic Cycle:
- Viruses attach to the host cell, penetrate the cell, utilize host machinery for biosynthesis, mature, and then release new virions.
- Lysogenic Cycle:
- Viral DNA gets integrated into the host's genome, leading to many cell divisions before activating the lytic cycle.