it's not something you "contract", that's for sure. but think about it: so much stuff is happening when we grow up, and develop. stutter manifests at some point in time, and it's not really down to a single cause. lots of stutterers have severe problems e.g. talking on the phone, or carrying out even very simple conversational tasks (presenting themselves by name, asking for directions, answering a question of some sort). but it fluctuates too. sometimes i can say very large sentences without a single hitch. other times, one tiny little word can cause me so much trouble that my expressing it comes to a complete stop and i have to stop momentarily, and then carry on. it's all relative, really...