Bed Bugs spread through hitchhiking. When you visit or place or business or a friends home or you go on a trip if their are bed bugs present and you sit down or lay down in that area all a bed bug has to do is crawl on to your luggage or clothing and hitch a ride back to your house.
We call them hitchhiking insects. That problem is now introduced into your environment and then if you have a friend come over and visit and let's say you have one or two bed bugs in your couch, they now have the problem at their house. If you don't have bed bugs and somebody visits your home and they do have them at their place you can also get them that way as well.