Various. There are at least 2 forms. Classic and new variant. Classic includes sporatic and familial) sporadic - which it is unknown but usually occurs later in life. Familial- which is passed genetically. Variant- mostly contamination (small percentage- ie contaminated medical instruments or eating contaminated foods).
Usually by eating. Contaminated meat (human or animal), through contaminated surgical instruments, pituitary derived growth hormone, very rarely through blood transfusion. Most cases are sporadic. See this site for more information. Http://www. Ncbi. Nlm. Nih. Gov/pubmedhealth/pmh0001792/.