KEY CONCEPT Infections can be caused in several ways. Figure: This bacteria lives in the lining of an esophagus, the tube that leads from the mouth to the stomach. At one time, it was thought that nothing could survive in an esophagus, but the entire digestive tract is home for many types of bacteria.

Viruses, bacteria, viroids, and prions can all cause infection. Any disease-causing agent is called a pathogen. Synthesize: Why are viruses, viroids and prions not included in the Linnaean system of biological classification? 1 nanometer (nm) = one billionth of a meter 100 nm eukaryotics cells 10,000-100,000 nm viroids 5-150 nm viruses 50-200 nm prokaryotics cells 200-10,000 nm prion 2-10 nm Figure: Although eukaryotic and prokaryotic cells can be microscopic, they are large in comparison to viruses, viroids, and prions.

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 Figure: Polyhedral viruses are many-sided, like the one shown here that causes foot-and-mouth disease in animals. (computer illustration)

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 enveloped (influenza) helical (rabies) polyhedral (foot-and-mouth disease) capsid nucleic acid lipid envelope surface proteins capsid surface proteins nucleic acid capsid nucleic acid lipid envelope Surface proteins

Bacteriophages are viruses that infect bacteria. capsid DNA tail sheath tail fiber Root words: -phage – means “to eat” So literally bacteriophage means bacteria eater.

Viruses cause many infectious diseases There are many examples of viral infections. common cold influenza

Viruses cause many infectious diseases There are many examples of viral infections. common cold influenza SARS

Viruses cause many infectious diseases There are many examples of viral infections. HIV HIV-infected white blood cell The body has natural defenses against viruses.

Vaccines are made from weakened pathogens. A vaccine stimulates the body’s own immune response. Vaccines prepare the immune system for a future attack. Vaccines are the only way to control the spread of viral disease.

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