Question: why does cancer exist and react in a bad way ????????

Rebecca Cook answered on 15 Jun 2013:

Cancer comes from cells in your body that have changed (mutated) over time and have begun to grow out of control. As you get older the risk of these mutations occuring increases and there are other risk factors such as smoking, exposure to UV light from the sun, other environmental factors and family history.

As cancer cells are growing out of control they also grow in the wrong places and start to get in the way of your healthy cells which will cause problems in that body part/organ resulting in the horrible effects of cancer. What makes cancer even worse is that cancer cells develop ways to get into your blood and travel around your body which allows the cancer to spread through your body, this process is called metastasis.