VirususVirususNon-living infectious agentsRequire host (parasites)Found everywhere.Infect organisms in every kingdomEdward Jenner-first vaccine for

smallpox 1796

Viral DiseasesViral Diseases

Viral diseases:–Smallpox, Polio, Measles,

Influenza, AIDS, Common Cold

Viruses are Infectious AgentsViruses are Infectious Agents

But are Host Specificand tissue specific

For Example: Cold Virus only infects the cells of the human upper respiratory tract.

Virus StructureVirus Structure

All contain these 2 structures :–1. Nucleic Acid (either RNA or DNA)

–2. Protein Coat (capsid)- Made of proteinEncloses Genetic material

Viral Body PlansViral Body Plans

Complex virus (bacteriophage)

Poylhedral virusHelical virus

Some are Enveloped: have an added phospholipid membrane on outside.

Examples: HIV, Rhinoviruses , Coronovirus Influenza

HIV is an Enveloped HIV is an Enveloped RetrovirusRetrovirus

Has RNA

Retrovirus=RNA—to DNA---To RNA---To protein

Mutates quickly.

HIV replication. 1.HIV replication. 1.

HIV ReplicationHIV Replication1. Binds to specific cell-Helper T cell (white

blood cell).2. Virus attaches, & viral RNA enters.3. Production -uses host materials to make DNA4. DNA enters nucleus; RNA made via

transcription. 5. RNA leaves nucleus, translation occurs.6. Assembly of new viruses. 7. Exits by budding and/or lysis of cell.

Fig. 20.11, p. 317

viral RNA

protein subunits of coat

18 nm diameter, 250 nm length

80-nm diameter lipid envelope; proteins span the envelope, line its inner surface, spike out above it

viral RNA

reverse transcriptase

viral coat (proteins)

100-120 nm diameter

DNA

protein coat

sheath

base plate

tail fiber

65-nm diameter head, 225-nm total length

Virus Reproduction-Virus Reproduction-

Takes ~20 min to make hundreds-to-thousands of new viruses

Various methods--- but 5 basic steps—as follows:

Virus Reproduction-5 basic steps.Virus Reproduction-5 basic steps.Lytic Cycle Lytic Cycle

1. Attach to host cell surface.2. Entry: whole virus OR just

DNA3.Replication-takes over & direct

replication of viral DNA & Proteins

4. Assembly-new viruses5. Release. By either Budding or

Lysis which results in cell death.

Defense Against VirusesDefense Against Viruses

1. ??Antibiotics—INEFFECTIVE— because they target cellular processes

(viruses are not alive—no cell/processes)

2. Vaccinations—Made of viruses which have been treated so they are harmlessCreate our own self-defense-build internal

antibodies against specific viruses.

2.2.

Defense Against VirusesDefense Against Viruses

3. Virus-Destroying Drugs. They destroy the virus BUT ALSO DESTROY HOST CELL–Example: AZT-slows down

AID’s; it’s toxic therefore side effects

Why Don’t Vaccines Always Why Don’t Vaccines Always Protect? Mutations –Protect? Mutations –

Become Unrecognizable ЖФξHIV, Cold Virus, Influenza change

oftenViral surface proteins mutate; some

VERY OFTENOf all the changeable viruses, HIV

changes the most!

Emerging Viral DiseasesEmerging Viral Diseases

Filovirus: A virus harbored in animals without harming them; then passed on to another animal who is harmed

FOR EXAMPLE: Ebola :30-90% mortality

hemorrhagic fever.

Fig. 20.6, p. 314

PrionDiseasePrionDisease

Other Infectious Agent--PrionsOther Infectious Agent--PrionsSmall abnormal forms of proteins necessary for operation of

neuronsproteinsProteins coagulate as large deposits in brain-Linked to human diseases

– Kuru– Creutzfeldt-Jakob disease (CJD)

Animal diseases– Scrapie in sheep– Bovine spongiform encephalopathy(mad cow disease)

Fig. 20.6, p. 314

PrionDiseasePrionDisease