A Brief Clinical Overview


Molecular Biology

Pox Postulates

Human Poxviruses

New Findings

Additional Information

Pathogen Cards


Human Poxviruses

  • Variola

  • Vaccinia 
  • Molluscum Contagiosum

Zoonotic Poxviruses

  • Cowpox
  • Monkeypox
  • Pseudocowpox
  • Orf
  • Yabapox 
  • Tanapox

Clinical Snapshots

Variola (Orthopoxvirus)

There are two main categories of smallpox, caused by two distinct viruses: variola major, the more severe and common strain that kills approximately 30% of individuals infected, and variola minor, or alastrim, that is relatively much more mild, with historical death rates of 1% or less.  Following an incubation period of 10-12 days, infected individuals enter the prodromal phase characterized by high fever, malaise, head & body aches, and often vomiting.  The smallpox rash emerges first as small red spots on the tongue and mouth, which quickly develop into sores that break opening and facilitate spread of the virus into the mouth and the throat.  At this point, the patient is highly infectious; the rash the progresses from macules to papules, vesicles (bumps are filled with thick fluid and depressed in the center), and pustules (raised bumps are firm to the touch)  as it spreads from the face to the distal extremities in a centrifugal distribution.  Approximately 10 days after the onset of rash, the pustules form crusts and then scabs.  Once the scabs fall off, the patient is no longer contagious, though survivors are often left with scars or pockmarks.

*Distribution of smallpox lesions compared to chickenpox lesions (left)

*Smallpox lesions on the arm (right)


 Vaccinia (Orthopoxvirus

The vaccinia virus, the live, attenuated virus in the smallpox vaccine, is a considerably milder relative of smallpox that can cause rash fever, and head and body aches in infected individuals.  Because the vaccine is live, individuals can inadvertently inoculate themselves - spreading the infection to other parts of the body - by touching the vaccination site before it has healed.


* Auto-inoculation following vaccination (above) 

*Reaction to smallpox vaccination (left)

Molluscum Contagiosum (Molluscipoxvirus)

Transmitted by direct contact or acquired sexually, molluscum is a fairly common skin condition that is found worldwide.  The incubation period is estimated to be between 2 weeks and 5 months, after which small nodular lesions (1-10 mm) appear.  The lesions, which then become umbilicated, are predominantly confined to the trunk, though they may also appear in the genital region in adults with sexually transmitted infections.  Within the lesions are 'molluscum bodies', ovoid structures that contain the infectious virions.  The majority of molluscum lesions are benign and self-limiting; while they often disappear spontaneously, recovery can be aided by cryotherapy, curettage, or treatment with salicylic acid or phenol.

*Small, pink or flesh-colored bumps with a dimpled center indicate a molluscum diagnosis 

Pseudocowpox (Parapoxvirus)

Cattle and prairie dogs are the primary hosts for pseudocowpox.  In humans, nodular lesions - 'milkers' nodes' - occur on the hands and face after contact with infected farm animals.  The lesions are painless and resolve over the course of several weeks in the absence of treatment.

Orf (Parapoxvirus)

Also known as 'sore mouth' or contagious ecthyma, orf is an infection primarily of sheep and goats that is found around the world.  Individuals who come into contact with the virus from an infected animal or equipment can get infected.  Humans infected with orf demonstrate granulomatous lesions on their hands that may be painful and can last for up to two months before resolving naturally.

Cowpox (Orthopoxvirus)

Cows, cats, and rodents serve as the main reservoirs of infection.  Most human cases are acquired from direct contact with teats of an infected cow.  In humans, the infection presents as lesions on the face and may resemble severe vaccinia infections.

Monkeypox (Orthopoxvirus)

This zoonosis is geographically confined to western and central Africa.  The key clinical features of monkeypox in humans are rash, fever, respiratory symptoms, and lymphadenopathy.  In 2003, the U.S. monkeypox outbreak was traced back to the importation of exotic pets from Africa, which then transmitted the virus to other pet animals such as prairie dogs. 

Tanapox (Yatapoxvirus)

Tanapox virus was first identified in Kenyan individuals during the two epidemics of 1957 and 1961,  Monkeys are the primary host of this zoonotic disease; infected humans present with vesicular skin lesions and febrile illness.


In African monkeys, yabapox causes subcutaneous growth whereas in accidental human infections, the clinical picture is one characterized by localized skin nodules that resolve spontaneously.