How is smallpox spread?
After entering the body the virus makes its way into the lungs and the lymph nodes. Once there, it starts the lytic cycle and begins to reproduce. On average this takes about 12 days and is called the incubation period. At the end of 12 days, the virus moves into the bloodstream. The body then mobilizes its resources to defend against the virus, causing fevers, muscle pain and stomach aches. In half of the cases it also leads to vomiting and in about 15% of the population it causes fainting.
At this point it is still possible to confuse smallpox infection with the common cold. Smallpox virus preferentially attacks skin cells and by days 14-15 smallpox infection becomes obvious. The attack on skin cells causes the characteristic pimples associated with the disease. The pimples tend to erupt first in the mouth, then the arms and the hands, and later the rest of the body. At that point the pimples, called macules, should still be fairly small. This is the stage at which the victim is most contagious.
By days 15-16 the condition worsens and the pimples grow into papules. These then fill up with pus, turning them into pustules. After the appearance of the pustules, the symptoms become vastly different. Route A is if the victim is going to survive the outbreak. The pustule will deflate in time (it is different depending on the patient), and will start to dry up, usually beginning on day 28. Eventually the pustules will dry off and start to flake off. Once all of the pustules flake off the patient is considered cured. If the patient is going to die, an entirely different set of symptoms starts to develop. First, bleeding will occur under the skin, making the skin look charred and black (this is known as black pox). Soon afterwards, bleeding begins in the organs. This is the final blow that kills the human that has the disease.
Smallpox is highly contagious. A person with smallpox becomes most contagious with the onset of rash, but because the early rash can be missed, they are presumed infectious from the time the fever starts. At this stage, the person is almost always very sick and not able to move around in the community. The infected person is contagious until the last smallpox scab falls off. Smallpox is most often spread by the respiratory secretions of people with smallpox to people who have close (< 6 ft) face to face contact. Less often it is spread through direct contact with smallpox lesions of the skin and mucous membranes, or through contact with materials (e.g., bedding, clothing) contaminated by such lesions or scabs. Rarely, it is spread through airborne means. Humans are the only known hosts; animals or insects do not spread the virus.