Viruses are tiny, infectious parasites that, unlike bacteria, fungi and protozoa, require a host to replicate. Viruses are typically sensitive to environmental conditions and cannot survive for long on surfaces. However, viruses can be highly infectious, easily transmitted and, being the smallest of the pathogens species, very difficult to eliminate with filtration.
A bacteriophage is a species of virus that invades bacteria and reproduces inside. Sometimes called phages, these viruses can be responsible for making bacteria pathogenic as well as a vehicle to combat superbugs. The most widely recognized examples of phage-encoded virulence factors are exotoxins, which is the major factor of several bacterial pathogens, including cholera, diphtheria and botulism.
Viruses can quickly spread through contaminated water and airborne droplets. Each year millions of school aged children are afflicted with Influenza (the “flu”), Rhinovirus (a cold), streptococcal pharyngitis (strep throat) and respiratory syncytial virus (RSV), with airborne transmission being a primary vector. During the 2016–2017 flu season, influenza infections resulted in an estimated 30.9 million people getting sick, 14.5 million going to a health care provider, and an estimated 600,000 people being hospitalized.
Norovirus is an example of a highly contagious, small, non-enveloped virus transmitted through food, water and touching contaminated surfaces. In the U.S., it is the leading cause of illness and outbreaks from contaminated food. Norovirus can spread quickly in closed places like daycare centers, nursing homes, schools and cruise ships.
A person with norovirus infection can shed billions of norovirus particles but it only takes as few as 18 viral particles to infect another person. During outbreaks, norovirus can spread in several different ways. For example, a person who is infected by eating contaminated food in a restaurant can spread the virus to household members through direct contact or indirectly by touching objects and surfaces.
The pathogenicity of norovirus makes testing and analysis of disinfection performance difficult and challenging. Instead, scientists rely on surrogate bacteriophages such as MS2 and Qβ.