Because it is easily spread from person to person through contact with body fluids.
Chickenpox is an infectious disease, not a genetic disease
Chickenpox is a viral infectious disease. It is not a migration.
Chickenpox is not a genetic disease. It's an infectious disease caused by varicella zoster virus.
To "carry" an infectious disease means to be infected without having symptoms; therefore, you can't "carry" chickenpox if you're not infected.
A carrier of chickenpox, like a carrier of any infectious disease, is someone who is infected and can transmit the germ but does not yet have symptoms. A chickenpox carrier is someone who's spreading the virus but does not yet have symptoms.
HIV, measles, chickenpox, the common cold, herpes, and mononucleosis are all viral infectious diseases.
Chickenpox is not an autoimmune disease. Chickenpox is a viral communicable disease.
Shingles is a viral disease related to chickenpox. It results from reactivation of the lifelong infection with the chickenpox virus. Chickenpox can cause myocarditis as a complication. There can be secondary bacterial infection of the skin. Rarely there can be viral encephalitis.
Chickenpox and mumps are vial infectious diseases that are vaccine preventable.
When people speak of a "carrier" of an infectious disease, they normally are referring to someone who is carrying the germ, can infect others, but has no symptoms. The period between getting the virus and getting symptoms is called the "incubation period" of an infection. The incubation period of chickenpox is 10-21 days, but is typically about two weeks. However, chickenpox is a virus in the herpes family that goes into remission after the patient recovers from chickenpox. The virus stays in your body. It is the same virus that causes shingles in some in later years. However, in this period of remission, the patient is not contagious and isn't, technically, a "carrier."
You catch shingles in old age because you had chicken pox as a child. Children catch chicken pox from other children with chicken pox or from old people with shingles. While no one inherits shingles, it may seem that way because children who caught chicken pox from their grandparents with shingles will give chicken pox to their grandchildren when they get old and get shingles unless the grandchildren get a vaccination for chickenpox.
Chickenpox is highly infectious, but it isn't 100% contagious. 90% of household contacts will be infected.