Chapter 27: Positive-Strand RNA Viruses

Positive-Strand RNA Viruses viruses, encompassing major families such as Picornaviridae, Togaviridae, Flaviviridae, Caliciviridae, and Coronaviridae, typically replicate within the host's cytoplasm and synthesize their proteins as large polyproteins that undergo proteolytic cleavage. The discussion highlights the Picornaviridae family, featuring acid-stable enteroviruses like poliovirus, which can transition from the gastrointestinal tract to the central nervous system to cause paralytic disease, and acid-labile rhinoviruses, the primary culprits behind the common cold. Significant attention is given to viral hepatitis, distinguishing between the acute, enterically transmitted Hepatitis A and E and the bloodborne Hepatitis C, which frequently leads to chronic conditions like cirrhosis and hepatocellular carcinoma. The chapter also delves into arboviruses, which are transmitted by arthropods like mosquitoes and ticks; these include the agents of yellow fever, dengue, West Nile, and Zika virus, the latter of which is associated with severe fetal developmental issues like microcephaly. Furthermore, the Rubivirus genus is examined for its role in rubella, emphasizing the critical importance of vaccination to prevent devastating congenital rubella syndrome. Finally, the Coronaviridae family is analyzed, noting the structural "corona" of surface spikes and their capacity to cause both mild upper respiratory infections and severe global health threats such as SARS and MERS. By integrating clinical significance, transmission pathways, and molecular replication strategies, this chapter provides a comprehensive overview of the pathogens responsible for a wide array of human illnesses.