KEY CONCEPT Infections can be caused in several ways.

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KEY CONCEPT Infections can be caused in several
ways.
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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 cells10,000-100,000 nm
viroids5-150 nm
viruses50-200 nm
prokaryotics cells200-10,000 nm
prion2-10 nm
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• 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

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• A prion is made only of proteins.

• causes misfolding of other proteins
• results in diseases of the brain like mad cow
  disease

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Viruses differ in shape and in ways of entering
host cells.

• Viruses have a simple structure.
• genetic material inside
• capsid, a protein shell on the outside
• some have a lipid envelope, a protective outer
  coat

helical(rabies)
polyhedral(foot-and-mouth disease)
enveloped(influenza)
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• Bacteriophages- viruses that infect bacteria.

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Are viruses living things?

• Most scientists say no
• Dont consume energy
• Cannot reproduce without a host cell
• Cannot synthesize their own proteins

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• Viruses enter cells in various ways.

• Bacteriophages (green) pierce host cells (brown)

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• Viruses enter cells in various ways.

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• Viruses enter cells in various ways.

• viruses of eukaryotes recognize certain receptor
  proteins and fuse with membrane
• enter by endocytosis

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Viruses cause two types of infections.

• A lytic infection causes the host cell
• to reproduce 100s of copies of the virus
• then bursts, releasing the new viruses.

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• A lysogenic infection does no immediate harm
• but may become infectious later

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Viruses cause many infectious diseases

• - common cold

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Viruses cause many infectious diseases

• common cold

• influenza

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Viruses cause many infectious diseases

• common cold

• influenza

• ebola

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Viruses cause many infectious diseases

• The body has natural defenses against viruses.
• Immune system cells
• T cells search out and destroy cells that have
  been infected stops viral reproduction.
• B cells
• make antibodies that bind to invading cells and
  mark them for destruction
• memorize the viruss surface proteins, then wait
  for a second infection
• virus will be destroyed before it can make you
  sick.immunity!

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Vaccines are made from weakened pathogens.

• A vaccine stimulates the bodys own immune
  response without actually getting the virus
• the only way to control the spread of viral
  disease.