If you have then you've been exposed to millions of germs. Sometimes we get sick, but amazingly enough, most of the time we don't. Our body's ability to fight off disease is due to the incredible immune system. Everyone gets sick once in a while, and beyond making us ill, there are still many diseases which can kill people.
Cancer is a disease which manages to defeat the immune system.
For years cancer immunologists assumed that the cells of the immune system simply couldn't recognize tumor cells. After all, if they could, cancer would be no more serious than the common cold. The many people who die each year from cancer seem to support the view that the immune system is blind to tumor cells. However, new experimental evidence indicates otherwise. As often happens in science the assumed or most obvious view is not always the correct one.