Every child of a parent with HD has a 50/50 chance of inheriting it, so it is ultimately a family disease.
It is caused by a fault in the gene that instructs the formation of a protein called huntingtin. We all inherit two copies of every gene – one from each of our parents. If the child of an HD parent inherits the faulty gene they will subsequently develop the disease at some time in their life. If the child of an HD parent inherits the gene that isn’t faulty they will never develop the disease and will not pass the disease to any of their offspring.