Acute infections followed by virus clearing Colds and respiratory infections

Cold viruses (rhinoviruses, adenoviruses, and coronaviruses) are spread as aerosols. Infection is localized within the nasopharynx, and recovery involves immunity against that specific virus serotype. The vast array of different cold viruses and serotypes ensures that there will always be another one to infect individuals. Although generally these types of respiratory diseases are mild, infection of an immune-compromised host or a person having complications due to another disease or advanced age can lead to major problems.


The epidemiology of influenza is an excellent model for the study of virus spread within a population. While symptoms can be severe, in part due to host factors, the virus infection is localized, and the virus is efficiently cleared from the host. Flu viruses have evolved unique mechanisms to ensure constant generation of genetic variants, and the constant appearance of new influenza virus serotypes leads to periodic epidemics of the disease. Some of these mechanisms are described in detail in Chapter 15, Part IV. The respiratory distress caused by most strains of flu virus is not particularly life threatening for healthy individuals, but poses a serious problem for older people and individuals with immune system or respiratory deficiencies. Some strains of the virus cause more severe symptoms with accompanying complications than others. At least one strain, the Spanish strain of 1918 (H1N1), caused a worldwide epidemic with extremely high mortality rates in the years immediately following World War I.


The disease caused by infection with smallpox (variola) virus is an example of a much more severe disease than flu, with correspondingly higher mortality rates. There are (or were) two forms of the disease: variola major and variola minor. These differed in severity of symptoms and death rates. Death rates for variola major approached 20%, and during the Middle Ages in Europe reached levels of 80% or higher in isolated communities. Virus spread was generally by inhalation of virus aerosols formed from drying exudate from infected individuals. The virus is unusually resistant to inactivation by desiccation and examples of transmission from contaminated material as long as several years after active infection were common.

The disease involves dissemination of virus throughout the host and infection of the skin. Indeed the pathogenesis of mouse pox described in the last chapter provides a fairly accurate model of smallpox pathogenesis. The virus encodes growth factors that were originally derived from cellular genes. These growth factors induce localized proliferation at sites of infection in the skin, which results in development of the characteristic pox (see Chapter 18, Part IV).

0 0

Post a comment