The initial symptoms of smallpox appear 12 to 14 days after getting exposed to smallpox virus. But the virus can survive in the body from 7 to 9 days before falling sick. This time is known as the incubation period. After this incubation period, sudden flu-like symptoms appear. They are fever, muscle aches, headache, fatigue, back pain, and sometimes vomiting. A few days later, flat red spots start appearing on the body. They may occur in the mouth, tongue and then spread to the skin. The face, arms, and legs get affected first followed by torso, hands and feet. Within a day or two, many of these spots can turn into small blisters filled with clear fluid. Later, these blisters get filled with pus. These sores are known as pustules. Scabs form 8 to 9 days later and fall off leaving deep pitted scars. Usually, small pox can spread from one person to another when the rash appears and until the scab falls off.