What is Isotope-131 and what harm can they do too people in the US?

I keep hearing about Isotope-131 and people on Yahoo freaking out saying it will cause cancer and kill alot of people and that it will get in water and contaminate the whole world. WTF DO I DO!? if Its reached Florida and South Carolina and Nevada I know its Reached here (Oklahoma) so will this Harm me? Or are the people on Yahoo in the Comments overreacting and making a big deal out of nothing?