Basic Virology. Martinez J. Hewlett

spring, new serotypes of respiratory viruses are communicated from the crew and passengers, and a “mini‐epidemic” occurs. As the virus passes through the population, resistance builds and infections decline to a low level.

      Source: Based on data originally published by Paul, J.H. and Freese, H.L. (1933). An epidemiological and bacteriological study of the “common cold” in an isolated Arctic community (Spitsbergen [sic]). American Journal of Hygiene7:517.

      In large populations, the rate of virus spread greatly surpasses the limitations of the generation of herd immunity, and the introduction of a novel pathogenic virus leads to epidemic spread of disease. The outbreak of severe acute respiratory syndrome (SARS) in China in the early part of the 2000s and its spread to Canada provide an important case study of this process, as well as providing examples of effective and ineffective public health measures set up to deal with it. The SARS virus is a member of the coronavirus family and distantly related to one that causes mild colds in humans. The virus appears to have been maintained in wild animal populations in southeast China and was introduced into humans in Guangdong Province and the city of Guangzhou (Canton) through the custom of using such animals as dietary delicacies. While human infection is characterized by flu‐like symptoms, the persistence, severity, and relatively high death rate suggested that this was a novel type of infection – a novel virulent form of influenza or an uncharacterized respiratory virus. Evidence suggests that the Chinese government, in hopes of avoiding loss of tourist and business travel revenues, suppressed news of this outbreak.

      The disease was spread by a physician who had treated infected individuals in China and then traveled to Hong Kong on business – as the first identified source of infection, he was termed the index case. He contaminated the registration desk of the hotel in which he was registered, and this desk served as a source of infection for a number of tourists from other parts of the world (including Toronto, Canada) who happened to be staying in the same hotel. The disease spread to individuals in Hong Kong and was eventually described and quarantined there, but not before other infected individuals traveled back to Canada and, in lesser numbers, to the United States.

      In Toronto, the index case of the local epidemic was a woman who infected her immediate family members upon returning from Hong Kong. She and one son subsequently died, but not before being admitted to the hospital where a physician treating them as well as other members of the hospital staff were infected. This illustrates a continuing conundrum of modern medicine – the concentration of individuals suffering from an infectious disease in a hospital can serve as a potent reservoir for the spread of that disease through the staff attending them and, subsequently, others. Such nosocomial infections are a major occupational hazard for hospital personnel as well as patients suffering other maladies, yet hospitals are obviously necessary for the treatment of the severely ill.

      The Canadian public health authorities were reluctant to initiate stringent quarantines for infected individuals in the hospital where the first patients were housed, and the hospital served as a source of infected individuals who spread the disease to others both through family and social contacts and through contact in other workplaces. In contrast, in the United States, the infection was initiated somewhat later. By that time sufficient information concerning the disease, its spread, and its control led to rapid quarantine of SARS patients, especially among health workers. These control methods were successful in the United States and Europe, as well as in Hong Kong, and the virus never spread beyond the first intimate contacts.

A fictionalized sequence of events based on the Canadian SARS outbreak is shown schematically in Figure 3.3. Without the intervention of public health and other government agencies, the spread would continue through a susceptible population for an extended period of time. Further, it is clear that rapid recognition of symptoms and effective quarantine of affected individuals are key to stopping spread. In the case of SARS, the suppression of information concerning its appearance until it was potentially out of control in Asia could have led to a widespread epidemic there and in neighboring countries.

Many have suggested that only the lucky fact that the SARS coronavirus (SARS‐CoV) is not particularly efficient at spreading between individuals saved us from a much more serious situation. Further, it has been argued that SARS provided a testing ground for public health response strategies, which worked reasonably well. Other examples of serious virus epidemics have not had as felicitous outcomes – for example, the HIV epidemic, which continues to grow and consume increasingly significant public health resources. Several other potentially lethal epidemics threaten the human population in the future, including a strain of avian influenza, and another coronavirus that is related to SARS‐CoV, Middle East respiratory syndrome coronavirus (MERS‐CoV), which caused a similar severe respiratory disease in the Middle East and is transmitted to humans from camels. Finally, the emergence of SARS‐CoV‐2 in 2019 with its more efficient human‐human transmission (than SARS‐CoV) resulted in the world‐wide pandemic of COVID‐19.

Figure 3.3 Fictionalized timeline of the spread of SARS virus following its introduction into Toronto, Canada, from Hong Kong in early 2003. Source: Data based on material presented on the CDC website (http://www.cdc.gov/ncidod/sars) and in the February 2004 issue of the Journal of Emerging Infectious Diseases, which was dedicated to studies on the SARS outbreak of late 2002–early 2003.

      General features of these diseases and the viruses that cause them will be discussed in Parts III and IV, and an overview of the potential threats of virus disease in the future will be briefly addressed in Part V. It suffices here to note that the dynamics of virus spread are not the problem; rather, it is coordinating political, public health, medical, and scientific resources targeted at the control of infection in a timely and efficient manner that is and will continue to be major challenges.

Factors affecting the control of viral disease in populations

      Generation of lasting immunity provides an effective means of controlling and even eradicating certain viral diseases. The antigenic stability of the smallpox virus and effective immunity against it allowed effective vaccination programs to eradicate the disease from the population. Polio and measles are current candidates for partial or total elimination from the population due to availability of effective vaccines. In addition, currently a program is underway to try to vaccinate wild populations of raccoons and other small carnivores against rabies with use of vaccine‐laced bait. It is hoped that such an approach will reduce or eliminate the growing incidence of rabies in US wild animal populations. Of course, the reason for this solicitude has little to do with the animals involved; rather, it is to afford protection to domestic animals, and ultimately to humans.

      Despite our considerable abilities, not all viral diseases can be readily controlled even under the most favorable economic and social conditions. Flu virus variants arise by genetic mixing of human and animal strains, and it is not practical to attempt a widespread vaccination campaign with so many variables. HIV remains associated with lymphatic tissue in infected individuals even when antiviral drugs effectively suppress virus replication. The intimate association of HIV with the immune system may make vaccination campaigns only partially effective. The ability of herpesviruses to establish latent infections and to reactivate suggests that a completely effective vaccine may be difficult if not impossible to generate.

      A major obstacle to the control of viral and other infectious diseases in the human population as a whole is economic. It costs a lot of money to develop, produce, and deploy a vaccine. Many of the nations most at risk of deadly infectious disease outbreaks are financially unable to afford effective