People are part of the environment. People shape the environment we live in, thus we are shaping each other. There isn't a thing a person does or say, that we hear, that doesn't affect us in one way or another. We are shaped by our senses and that is the things we see, hear, touch, feel, smell or taste. If we see a horrific crime, it changes how we percieve that type of person or character. If we hear thunder and lightning at night we get scared and fear the dark. if we are physically abused we are changed by violence. If we smell a bad stinch we tend not to go near it. If we taste something and it makes us sick we vere away from it. Everything we are exposed to affects us in one way or another.
Some things and people are more aggresive in nature and have a deaper impact on the environment, than any other stimuli we may be exposed to in life. As far as people are concerned, a prime example is Adolf Hitler. If any person in the history of mankind as we know it has had an impact on the entire world its that man. Research his life and the things he did and you will find your answer.