Chapter 13: Viruses, Viroids, and Prions

Their structure includes a nucleic acid core (DNA or RNA, but never both), a protein coat (capsid), and sometimes an envelope with spikes. Viral morphology is categorized as helical, polyhedral, enveloped, or complex, with examples provided such as influenza, herpesviruses, and bacteriophages. Virus classification is explained using the ICTV system, based on nucleic acid type, replication strategy, and morphology. The chapter discusses viral host range and specificity, emphasizing that viruses infect specific cell types in particular species. Bacteriophages—viruses that infect bacteria—are explored in detail, particularly the lytic cycle (resulting in host cell lysis) and the lysogenic cycle (where viral DNA integrates into the host genome as a prophage). Key terms like eclipse period, burst time, and burst size are defined within the context of one-step growth curves. Animal virus replication is also described, detailing the steps of attachment, entry (via fusion or endocytosis), uncoating, biosynthesis, maturation, and release. DNA and RNA virus replication strategies are compared, and retroviruses like HIV are explained in terms of reverse transcription and integration into host DNA. Oncogenic viruses such as HPV, EBV, and Hepatitis B are highlighted for their role in cancer development through insertional mutagenesis and interference with tumor suppressor genes. Viroids are introduced as short pieces of naked RNA that cause disease in plants, while prions are infectious proteins responsible for fatal neurodegenerative diseases such as Creutzfeldt-Jakob disease and mad cow disease (BSE). Prion replication through abnormal protein folding is discussed, as well as their resistance to standard sterilization methods. The chapter concludes by emphasizing the significance of these nonliving pathogens in medicine, agriculture, and public health.