The varicella-zoster virus, more commonly known as "chickenpox," incubates in the human body before it begins to spread from one person to another. The incubation period generally starts about two weeks after contact with the virus in its mature state, but can begin as soon as 10 days or as late as 21 days after contact. After the incubation period has begun to wind down, the disease is often capable of being transmitted outside of the host's body--often before visible blisters and bumps appear.
After the chickenpox virus has matured in its human host, the extremely contagious virus may be spread to other individuals through physical contact--both through indirect and direct contact. Examples of direct physical contact include touching a blister or the liquid from a blister of an infected person--even the mucus and saliva are contagious. Indirect physical contact also causes the virus to spread. Mucus, saliva and blister residue left on things like clothing and furniture can cause the virus to spread, indirectly, to a new host and start a new incubation cycle.
Physical contact isn't the only cause for the spread of chickenpox. The chickenpox virus is often spread "through the air by inhaling respiratory droplets" from and infected person. Involuntary body functions, like sneezing and coughing, cause the virus to spread through the air. A single sneeze can release the virus into the air at rates up to "100 miles per hour, and the bacteria can travel anywhere from three feet away to 150 feet," according to a report on WKOW-TV.