shih-tzu agressive dogs anyone?

I have heard alot about shih-tzu but I don't think that I have ever heard about agressive shih-tzu...I think what I have heard about them is that since they are kind of a small dog and the breed of dog they are it is easier to for them to get sick(I am not sure that its that dog it just rings a bell whenever I saw the name shih-tzu)I think I have known people that have had them before and they never really had a problem with them being mean or anyhting like that i think its really how you raise the dog!