ABSTRACT
Viruses are the cause of all the major pandemics that have occurred since 1900. To understand the intrinsic factors that determine this impressive pathogenicity, this chapter follows the historiography of the emergence of viruses as agents of disease, beginning with their original recognition at the end of the nineteenth century when the germ theory of disease was hailed as the etiology of all diseases. From the original indirect methods proving viral disease etiology through to electron microscopy, allowing visualization of viral structure, to molecular genetics, uncovering the importance of ongoing mutation as a powerful virulence tool, the chapter uncovers the qualities that make viruses such powerful pathogens. The discovery of the process of viral replication is described in which metabolically inert viruses take over the function of host cells, inserting their genetic material and taking over the cell’s functions. Newly created virus components are assembled into thousands of new viruses which then emerge to infect new cells. The importance of host cell surface receptors for recognition and binding as the mandatory, initiating step in cell infection is addressed, as are the many options for viral transmission. The important roles of cross-species infection, animal reservoirs, vectors and vertical transmission within hosts are described as part of the extensive armamentarium that makes viruses such formidable enemies.
Since just the turn of the last century, we have faced the continuing global spread of HIV/AIDS, SARS (Severe Acute Respiratory Syndrome), a global influenza pandemic with a newly emerged H1N9 strain, the global spread of Zika with its devastating pre-natal consequences, overwhelming and ongoing Ebola outbreaks, and now the growing COVID-19 pandemic due to a novel coronavirus. This chapter illuminates the unique biology of viruses as a critical element in determining the progression from local outbreak to global pandemic.
