As we have seen, cancer cells are created from normal precursor cells via an accumulation of genetic damage. The mechanisms by which the changes are induced is varied. In a broad sense, the agents of genetic change (mutation) fall primarily into the categories described below and are discussed in depth on the following pages.
Spontaneous Mutations - mutations resulting from unrepaired replication errors or random molecular events
Induced Mutations - mutations caused by things such as radiation, chemicals, chronic inflammation, and oxygen radicals.
Viruses - a significant percentage of cancers are caused by viruses. Viruses can directly damage DNA, acting like mutagens, they can alter the behavior of the infected cells or they can induce changes in nearby cells, indirectly leading to cancer.
Transposons - transposons are short DNA sequences that can 'jump' around the geneome. Landiing in an important gene can lead to cancer.
The animation below shows some of the causes of DNA damage (mutation).