**2. The current WHO Priority List**

The current WHO Priority disease list (**Box 1**) consists of viral diseases and Disease X, the latter being a placeholder that is always included in these lists. Disease X is used for the next unknown pathogen with the potential to cause a serious international epidemic. The viral families represented include *Arenaviridae* (Lassa fever virus); *Bunyaviridae* (Crimean-Congo hemorrhagic fever virus and Rift Valley fever virus); *Coronaviridae* (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2] causing COVID-19, Middle East respiratory syndrome coronavirus [MERS-CoV], and severe acute respiratory syndrome coronavirus [SARS-CoV]); *Filoviridae* (Ebola virus and Marburg virus]; *Flaviviridae* (Zika virus); and *Paramyxoviridae* (Nipah virus and henipaviruses).


Some of the characteristics of these viruses and their differences and commonalities are displayed in **Table 1**. Interestingly, these are each relatively

*a CCHFV, Crimean-Congo hemorrhagic fever virus; MERS-CoV, Middle East respiratory syndrome coronavirus; RVFV, Rift Valley fever virus; SARS-CoV, severe acute respiratory syndrome coronavirus; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; , ambisense; , negative sense; +, positive sense; ss, single-stranded; segments (1) equates to a non-segmented genome. b Suspected primary host [14].*

#### **Table 1.**

*Characteristics of World Health Organization Priority List viruses.*

large, lipid-enveloped viruses having single-stranded RNA genomes. Primary host infection of hemorrhagic viruses can be through insect vectors (arboviruses and flaviviruses), eating contaminated meat (filoviruses), consuming products in contact with bodily fluids of bats or pigs, such as blood, urine, nasal, respiratory droplets, and saliva (Nipah or henipaviruses), or exposure to contaminated rodent urine (Lassa virus). Once a human host is infected, the virus may be transmitted through contaminated bodily fluids and/or respiratory droplets. Non-hemorrhagic viruses such as SARS-CoV, SARS-CoV-2, and MERS-CoV are believed to be spread primarily by respiratory aerosols/droplets, although fomite transmission is also believed to play a role [10]. Case mortality rates vary, with SARS-CoV-2 having perhaps the lowest (2.1%), and Ebola Zaire virus among the highest (90%). These viruses retain infectivity for hours to days after being deposited experimentally on non-porous surfaces [10].

The relatively high lethality of these viral diseases and the ability of the viruses to survive on surfaces [10] inform the need for effective hygiene interventions for interrupting the cycle of infection. Since these viruses have been placed on the WHO Priority List, one might assume that not much is known about virucidal efficacy of microbicides intended for surface hygiene, hand hygiene, and for rendering contaminated test samples safe for use in diagnostic testing for these viruses. In the remainder of this chapter, we review the information that is available on this topic, in order to address this assumption for the reader. The literature on SARS-CoV-2 virucidal efficacy is being updated continually, so the information presented in this chapter on SARS-CoV-2, specifically, should be considered a snapshot taken at the present point in time (i.e., September 2021).

#### **Figure 1.**

*Hierarchy of susceptibility of pathogens to microbicidal active ingredients. Certain formulated microbicides may include combinations of active ingredients, resulting in synergistic virucidal efficacy greater than that displayed by the individual active ingredients ([15] modified from Sattar [8]).*
