Why is it difficult to establish a link between cancers or other illnesses and environmental exposures?

The complex nature of cancer makes it inherently challenging to identify, interpret, and address cancer clusters. Cancer is a term describing different diseases that share a similar characteristic: uncontrollable cell growth and division. As a group, cancers are very common. Cancers are the second leading cause of death in the United States, exceeded only by diseases of the heart and circulatory system. One of every four deaths in the United States is due to some form of cancer.  Read more information about cancer clusters.