1. The Infectious Agents of Disease
Intimate Strangers
The Infectious Agents of Disease

2. Infectious Agents
Viruses
Bacteria
Protozoa

3. Viruses Sub microscopic “primitive life form” Can not self replicate
Obligate intracellular parasite (totally dependent on host cells for replication)
Unaffected by antibiotics
Made of a core of DNA or RNA surrounded by a protein coat

4. Virus-From the Latin for ‘poison’
Viruses straddle the definition of life. They lie somewhere between molecular complexes and very simple biological entities. Viruses contain some of the structures and exhibit some of the activities that are common to organic life, but they are missing many of the others. In general, viruses are entirely composed of a single strand of genetic information encased within a protein capsule. Viruses lack most of the internal structure and machinery which characterize “life”. In order for a virus to replicate it must infect a suitable host cell.

5. Basic Structure of a Virus
Capsid- protein coat containing DNA or RNA, and enzymes
Envelope- similar to a membrane surrounding the capsid in some viruses; (proteins, lipids, and glycoproteins make up the envelope)

6. Nucleic Acid-DNA or RNA?
DNA Viruses: Smallpox, Chickenpox, Herpes, HPV, Adenovirus (can cause the cold), and Hep B
RNA Viruses: Measles, mumps, rabies, ebola, flu, polio, rhinovirus
**Retrovirus: contains reverse transcriptase to incorporate RNA DNA, then into host DNA Example: HIV
*Both types can replicate in both lifecycles. Some can lie dormant (latent) Ex: HIV (in the host DNA), Chicken Pox and Herpes (in neurons only)

7. They may look similar….

8. Virus Structure Polyhedral Helical
(Capsid has 20 triangular faces and 12 corners)
Ex: Influenza Virus
Helical
(Capsid is a coiled spring)
Ex: Tobacco Mosaic Virus

9. Bacteriophage Virus that infects bacteria
Structure is unique: polyhedral head on a helical tail with DNA inside the head
Has “legs” that land on the surface of the bacteria to infect it

11. Viral Roles Viruses can be harmful. Viruses may be helpful.
Virulent - disease causing.
Temperate - not immediately disease causing.
Viruses play a role in causing some cancers.
Viruses may be helpful.
A temperate virus can be used to cause genetic variation within a host population. (tulips)
Can be used to introduce genes into defective DNA “biologic vectors” (genetic engineering)

12. Viral Infection A. Virus injects DNA/RNA into host cell
Lytic Infection
A. Virus injects DNA/RNA into host cell
B. Host begins making viral proteins (can’t tell the difference b/t viral and its own nucleic acid)
C. Assembles new viruses
D. Lyses (bursts) the cell releasing new viruses, some can “bud” off the surface
Ex: T4 Bacteriophage, flu, cold

13. Viral Infection A. Virus injects DNA/RNA into host cell
Lysogenic Infection
A. Virus injects DNA/RNA into host cell
B. Once DNA is present, forms a prophage (literally embeds itself into the hosts’ DNA)
C. Remains dormant for a period of time until environmental factors activate it into the lytic cycle (chemicals,radiation,illness, stress, etc)
D. Goes directly into the lytic cycle
Ex: Herpes Simplex Virus

15. Virus Life Cycle Animations
1= HIV narrated
2= Lytic vs Lysogenic
3= Flu Attack

16. Just how big is a virus?

17. Some Diseases Caused by Viruses
Plants Animals Humans Tobacco mosaic Foot and Mouth Common cold Tomato bushy Rabies German measles
Alfalfa mosaic Distemper influenza Cold Sores
Sugar beet curly top Cowpox Polio
Dwarfism in rice Herpes, AIDS,
Hepatitis
Smallpox