Some concepts about cancer can be helpful when trying to understand suspected cancer clusters:Cancer is the uncontrolled growth and spread of abnormal cells anywhere in the body. However, cancer is not just one disease; it is actually an umbrella term for at least 100 different but related diseases Each type of cancer has certain known and/or suspected risk factors associated with it Cancer is not caused by injuries, nor is it contagious. It cannot be passed from one person to another like a cold or flu virus Cancer is almost always caused by a combination of factors that interact in ways that are not yet fully understood Carcinogenesis (the process by which normal cells are transformed into cancer cells) involves a series of changes within cells that usually occur over many years. More than 10 years can go by between the exposure to a carcinogen (any substance that causes cancer) and a diagnosis of cancer, which makes it difficult to pinpoint the cause of that cancer Cancer is more likely to occur as people get older; because people are living longer, more cases of cancer can be expected in the future. This increased life expectancy may create the impression that cancer is becoming much more common, even though an increase in the number of cases of cancer is related in large part to the growing number of elderly people in the population Some racial and ethnic groups have higher rates of cancer than other racial and ethnic groups. Such differences may be due to multiple factors, such as late stage of disease at diagnosis, barriers to health care access, history of other diseases, biologic and genetic differences, health behaviors, and other risk factors Cancer, in general, is common. More than 17 million new cases of cancer have been diagnosed since 1990
This answer is based on source information from the National Cancer Institute.