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  • Viral infections of skin and mucosa produce a wide spectrum of clinical manifestations.
  • Viruses that cause febrile illness with exanthems are usually self-limited, with primary infection conveying lifetime immunity.
  • Viruses such as human papillomavirus (HPV) and molluscum contagiosum virus (MCV) colonize the epidermis of most individuals without causing any clinical lesions.
    • Benign epithelial proliferations, i.e., warts and molluscum, occur in some colonized persons, are transient, and eventually resolve without therapy.
    • In immunocompromised individuals, however, these lesions may become extensive, persistent, and refractory to therapy.
  • The eight human herpesviruses often have asymptomatic primary infection but are characterized by lifelong latent infection.
    • In the setting of immunocompromise, herpesviruses can become active and cause disease with significant morbidity and mortality rates.


  • The poxvirus family is a diverse group of epitheliotropic viruses that infect humans and animals.
    • The genera of poxviruses that infect humans include orthopoxvirus, parapoxvirus, molluscipoxvirus, and yatapoxvirus (Table 27-1).
    • Only smallpox virus (SPV) and molluscum contagiosum virus (MCV) cause natural disease in humans.
    • Small pox (SPV) and monkeypox virus typically cause systemic disease with rash; other poxviruses cause localized skin lesions.
    • Other poxviruses are associated with zoonotic infections.
  • Poxviruses are the largest of all animal viruses and have a double-strand DNA genome.
    • They are the only DNA viruses that replicate in cytoplasm, where accumulated viral particles form eosinophilic inclusions, or Guarnieri bodies, visible by light microscopy (200–400 μm).
    • Poxviruses appear as brick-shaped or oval virus particles by electron microscopy.
    • The nucleosome contains double-strand DNA, which is surrounded by a membrane.
    • The outer surface of the lipoprotein bilayer has surface tubules that are randomly arranged and give the virion its characteristic textured appearance.
  • Smallpox, or variola, has been eradicated as a naturally occurring infection.
    • Cowpox is an infection of cattle caused by cowpox virus.
    • The origins of vaccinia virus, which is used to immunize humans against smallpox, are uncertain. It may be derived from variola virus, cowpox virus, or be a hybrid of the two.
  • MCV colonizes the skin of many healthy individuals, causing molluscum contagiosum, self-limited epidermal proliferations that resolve spontaneously.
  • Human orf and milker's nodules are zoonotic infections that can sometimes occur in exposed humans.
  • Other poxviruses that are zoonoses in animal hosts (monkeys, cows, buffalo, sheep, goats) can also infect humans.
  • Poxviruses cause toxic effects on cells, which result in cell rounding and clumping, degeneration of cell architecture, and production of cytoplasmic vacuoles.
  • Different poxviruses are capable of producing a localized, self-limited infection by inoculation to the skin (e.g., orf) or a fulminant systemic disease (e.g., variola).
  • The same virus can affect different species in different ways.

Table Graphic Jump Location
Table 27-1 Poxviruses that Infect Humans* and Cause Disease

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