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 10,000-100,000 nm viroids 5-150 nm viruses 50-200 nm prokaryotics cells 200-10,000 nm prion 2-10 nm

A prion is made only of proteins. causes misfolding of other proteins results in diseases of the brain A viroid is made only of single-stranded RNA. causes disease in plants passed through seeds or pollen

Viruses differ in shape, genetic material, and in ways of entering host cells. non-living pathogen can infect many organisms Viruses have a simple structure. genetic material (either DNA or RNA) capsid, a protein shell maybe a lipid envelope, a protective outer coat

Viral Shapes: depends on proteins of capsid 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

A bacteriophage is a virus that infects a bacterium. capsid DNA tail sheath tail fiber HEAD collar TAIL Base plate

Viruses enter cells in various ways. bacteriophages pierce host cells viruses of eukaryotes enter by endocytosis or fuse with the membrane

LYTIC CYCLE