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• Viruses are tiny infective agents that depend on the host for survival. They are 10-300nm and only visible under an electron microscope.
• Each virus contains DNA or RNA but never both. The virus has a protein coat that protects it from harsh environment.
• Bacteriophage structure

  A) head

  B) DNA

  C) Tail

  
• Enveloped viruses – has lipid and protein capsule. Has strtucture for attaching to host cell. A virus without a capsule is called naked.
• Virus propagation stage – 6 steps.

  Attachment, penetration, uncoating, replication, assembly and release.
• Attachment - when virion and specific receptor sites interact.

  Penetration – viral particle taken into cell by endocytosis and endosomes appear in the cell.

  Uncoating – capsid and viral nucleic acid is seperated

  Replication – virus components are synthesised and not visible in cell as they are small

  Assembly – new viruses formed due to viral genomes and capsid joining.

  Release – viruses burst the cell and leave through exocytosis (budding)
• Viruses propagation effects – interrupts normal cell cycle. Forms benign or malignant growth and is asymptomatic.

  Viral virulence – when viruses infect certain species with different types of ranges (broad or narrow)
• Viral infections can be warts, TMV plants and rabbies.
• Viruses examples – HPV, rubella, coronavirus,
• Viruses effect on host cell –

  There are 2 types of infections lytic and lysogenic.
• Lytic infections – cell ruptures when enzyme produced erode bacterial cell wall. (virulent viruses)

  Lysogenic infection – viral DNA/RNA coexists with the cell as it becomes incorporated into the genome. (lysogenic viruses)

  Lyosgenic becomes activated into the lytic phase.
• Viral infections treatment – it is difficult due to the virus using the host cell own process for propagation.

  Viral chemotherapy is used.

  Compounds such as Acylovir (cold sore creams and herpes virus) and Interferons (AIDS treatment) are used.

  Current approaches to treatment is early detection and prevention.
• Subviral particles – are particles that are infectious agents that are smaller than viruses. Examples are viroids and prions.
• Prions – means proteinaceous infectious particle. Smaller than viroid. It is an infectious agent with nucleic acid.
• Viroids – shorts strands of RNA. Has 250-400 circular nuclotides. No coding for proteins. No capsid. Infects plant cells and formed by the copying of RNA polymerase
• Prions are involved in spongioform encephalopathies. They usually have a fatal outcome and are slow viruses.

  In cattle they form mad cow disease and scrapie in sheep.
• Prions in humans – forms kuru disease (spongioform encephalopathy)

  It affects the nerves and brain and gives CJD