1. KEY CONCEPT Infections can be caused in several ways.

2. Viruses and Prokaryotes
Chapter 18

3. How are infections causes?

4. Viruses, bacteria, viroids, and prions can all cause infection.
Any disease-causing agent is called a pathogen.
1 nanometer (nm) = one billionth of a meter
100 nm
eukaryotics cells 10, ,000 nm
viroids nm
viruses nm
prokaryotics cells ,000 nm
prion 2-10 nm

5. A virus is made of DNA or RNA and a protein coat.
non-living pathogen
can infect many organisms
A viroid is made only of single-stranded RNA.
causes disease in plants
passed through seeds or pollen

6. A prion is made only of proteins.
causes misfolding of other proteins
results in diseases of the brain

7. Think back about computer viruses… At what point does the virus become dangerous?

8. How do viruses enter the human populations??

9. viruses can only replicate after they have infected host cells
Packaged genes that move from host to host

10. Viruses differ in shape and in ways of entering host cells.
Viruses have a simple structure.
genetic material
capsid, a protein shell
maybe a lipid envelope, a protective outer coat
polyhedral (foot-and-mouth
disease)
enveloped (influenza)
helical (rabies)
capsid
nucleic acid
lipid envelope
surface proteins
capsid
surface proteins
nucleic acid
capsid
nucleic acid
lipid envelope
Surface proteins

13. Bacteriophages infect bacteria.
capsid
DNA
tail sheath
tail fiber

14. Viruses enter cells in various ways.

15. Flu video

16. bacteriophages pierce host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x

17. Viruses of eukaryotes also fuse with membrane

18. Viruses cause two types of infections.
A lytic infection causes the host cell to burst.
host bacterium
The bacterophage attaches and injects it DNA into a host bacterium.
The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells.
The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages.
The viral DNA forms a circle.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.

19. A lysogenic infection does no immediate harm.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Although the prophage is not
active, it replicates along with
Many cell divisions produce a
colony of bacteria infected
with prophage.
The prophage may leave the
host’s DNA and enter the
lytic cycle.

20. What is the difference between getting a vaccination shot and taking an antibiotic?

21. Viruses cause many infectious diseases

22. There are many examples of viral infections.
common cold
Over 200
known types

23. Influenza (FLU)

24. SARS
Severe acute
Respiratory syndrome

25. HIV
Causes AIDS
HIV-infected white
blood cell

26. Some viruses like the flu can spread very quickly and cause the and epidemic
Rapid outbreak
More than 20 % of the population is infected
Viruses usually infect humans but their have also been viruses that have infected pigs, birds, whales horses and seals
Some of these have jumped species (Bird, swine

27. How do we protect ourselves?

28. The body has natural defenses against viruses.
Skin
Outer skeleton
Cell wall
Viruses can penetrate the skin through and opening
Once inside the body the virus finds a target cell, the cell “welcomes” it in

29. Vaccines – stimulates immune system
Made of the weakened or parts of the virus
Only way of preventing the spread of viral infections