The first symptoms of smallpox usually appear 10 to 14 days after you're infected. During the incubation period of seven to 17 days, you look and feel healthy and can't infect others.

Following the incubation period, a sudden onset of flu-like signs and symptoms occurs. These include:

• Fever
• Overall discomfort
• Headache
• Severe fatigue
• Severe back pain
• Vomiting, possibly

A few days later, flat, red spots appear first on your face, hands and forearms, and later on your trunk. Within a day or two, many of these lesions turn into small blisters filled with clear fluid, which then turns into pus. Scabs begin to form eight to nine days later and eventually fall off, leaving deep, pitted scars.

Lesions also develop in the mucous membranes of your nose and mouth and quickly turn into sores that break open.

Smallpox is caused by infection with the variola virus. The virus can be transmitted:

• Directly from person to person. Direct transmission of the virus requires fairly prolonged face-to-face contact. The virus can be transmitted through the air by droplets that escape when an infected person coughs, sneezes or talks.
• Indirectly from an infected person. In rare instances, airborne virus can spread farther, possibly through the ventilation system in a building, infecting people in other rooms or on other floors.
• Via contaminated items. Smallpox can also spread through contact with contaminated clothing and bedding, although the risk of infection from these sources is less common.
• As a terrorist weapon, potentially. A deliberate release of smallpox is a remote threat. However, because any release of the virus could spread the disease quickly, government officials have taken numerous precautions to protect against this possibility, such as stockpiling smallpox vaccine.

Most people who get smallpox survive. However, a few rare varieties of smallpox are almost always fatal. These more-severe forms most commonly affect pregnant women and people with impaired immune systems.

People who recover from smallpox usually have severe scars, especially on the face, arms and legs. In some cases, smallpox may cause blindness.