The incubation period between contraction and the first obvious symptoms
of the disease is around 12 days. Once inhaled, variola major virus invades the
oropharyngeal (mouth and throat) or the respiratory mucosa, migrates to
regional lymph nodes, and begins to multiply. In the initial growth phase the
virus seems to move from cell to cell, but around the 12th day, lysis of many
infected cells occurs and the virus is found in the bloodstream in large
numbers (this is called viremia),
of the disease is around 12 days. Once inhaled, variola major virus invades the
oropharyngeal (mouth and throat) or the respiratory mucosa, migrates to
regional lymph nodes, and begins to multiply. In the initial growth phase the
virus seems to move from cell to cell, but around the 12th day, lysis of many
infected cells occurs and the virus is found in the bloodstream in large
numbers (this is called viremia),
and a second wave of multiplication occurs in the spleen, bone marrow, and
lymph nodes. The initial or prodromal symptoms are similar to other viral
diseases such as influenza and the common cold: fever of at least 38.5 °C
(101 °F), muscle pain, malaise, headache and prostration. As the digestive
tract is commonly involved, nausea and vomiting and backache often occur. The
prodrome, or preeruptive stage, usually lasts 2–4 days. By days 12–15 the first
visible lesions—small reddish spots called enanthem—appear on mucous membranes of the mouth, tongue, palate,
and throat, and temperature falls to near normal. These lesions rapidly enlarge
and rupture, releasing large amounts of virus into the saliva.
Smallpox virus preferentially attacks skin cells, causing the characteristic
pimples (called macules) associated with the disease. A rash develops on the
skin 24 to 48 hours after lesions on the mucous membranes appear. Typically the
macules first appear on the forehead, then rapidly spread to the whole face,
proximal portions of extremities, the trunk, and lastly to distal portions of
extremities. The process takes no more than 24 to 36 hours, after which no new
lesions appear. At this point variola major infection can take several very
different courses, resulting in four types of smallpox disease based on the Rao
classification: ordinary, modified, malignant (or flat), and hemorrhagic.
Historically, smallpox has an overall fatality rate of about 30%; however, the
malignant and hemorrhagic forms are usually fatal.
pimples (called macules) associated with the disease. A rash develops on the
skin 24 to 48 hours after lesions on the mucous membranes appear. Typically the
macules first appear on the forehead, then rapidly spread to the whole face,
proximal portions of extremities, the trunk, and lastly to distal portions of
extremities. The process takes no more than 24 to 36 hours, after which no new
lesions appear. At this point variola major infection can take several very
different courses, resulting in four types of smallpox disease based on the Rao
classification: ordinary, modified, malignant (or flat), and hemorrhagic.
Historically, smallpox has an overall fatality rate of about 30%; however, the
malignant and hemorrhagic forms are usually fatal.
