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Plate 31 Introduction to Viruses Diseases Caused by Viruses • Smallpox • Chickenpox • Influenza (“the flu”) • Herpes • Polio • Rabies • Ebola • Human Papillomavirus (HPV) • Human Immunodeficiency Virus Virus Size Viruses can only seen by electron microscopes Average size is measured in nanometers (nm) Bacteria & human cells are measured in micrometers (1000 times larger than nm) Size Comparison E. coli is 3000 nm in size Chlamydia trachomatis is 300 nm in size Rabies virus is 200 nm in size Two General Shapes • Viruses can come in a wide variety of shapes, but generally they are either helical or icosahedral Helix Icosahedron Viral Components Nucleic Acid – either DNA or RNA that controls viral replication Capsid – outer protein coat for protection (made of capsomeres) Lipid envelope – flexible outer membrane, from host cell Viral Genetic Material DNA RNA (retroviruses) Lipoprotein Coat Cell membrane from killed host cells Can protect against host immune system “Cloak of invisibility” Obligate Intracellular Parasites Viruses are sometimes referred to as “obligate intracellular parasites” Obligate – needed / necessary Intra – within Cellular – relating to cells Parasite – one organism harms another in a long-term interaction