How does cancer usually begin to develop in the body?

Cancer usually begins with genetic mutations in a single cell. These mutations can cause the cell to grow and divide uncontrollably, forming a tumor. Over time, the tumor can grow and spread to other parts of the body.

Most cancers are thought to arise from somatic mutations, which are mutations that occur in cells other than egg and sperm cells. Somatic mutations can be caused by exposure to environmental factors, such as ultraviolet radiation from the sun or certain chemicals. They can also occur spontaneously, due to errors in DNA replication.

Genetic mutations that lead to cancer typically affect genes that control cell growth and division. These genes are called oncogenes and tumor suppressor genes.

Oncogenes are normally involved in promoting cell growth and division. When they are mutated, they can become constitutively active, meaning that they are constantly turned on, leading to uncontrolled cell growth.

Tumor suppressor genes, on the other hand, normally serve to slow down cell growth and division and to repair damaged DNA. When they are mutated, they can lose their ability to perform these functions, allowing cells to grow and divide uncontrollably.

In rare cases, some cancers result from germline mutations, which are mutations that occur in sperm or egg cells. These mutations are inherited and present in cells at the earliest stages of development.