Cancer is the result of uncontrolled growth by the cells in a particular tissue, forming a tumour that takes away resources from the rest of the body.
It is probably a mistake to look for a single cause for all cancers, because cancers themselves are different, and so probably have many different causes.
Cancers all have one thing in common: some control that would normally stop cells from multiplying forever has broken down, so a tumour is able to grow.
Chemicals which cause mutations often cause cancers as well, because these chemicals cause mutations in the protective genes that trigger apoptosis.
Chemicals which cause mutations often cause cancers as well. Cancers are usually prevented until a protective gene breaks down when it is damaged or mutated.
Cancers are often caused by a mutation in one of the protective genes that normally trigger a faulty cell to self-destruct for the benefit of the organism.
Our immune system does not protect us against cancer, although the normal operations of apoptosis behave in many ways as a form of immune defence.
Even though the immune system as such does not usually attack cancers, our immune systems can be artificially sensitized to attack some cancers.
Some cancers spread by metastasis, a process which involves cancerous cells spreading through the body and establishing secondary cancers at new sites.
In 1775 Sir Percival Potts noted that cancers of the nasal cavity and scrotum are common in chimney sweeps, that environmental factors can cause cancer.
Some chemicals released in the environment such as exhaust gases and even apparently harmless material like sawdust, may cause cell damage and lead to cancers.
People can inherit a greater probability of getting some cancers, including skin cancers (more common in Celts) and some of the colonic cancers.
Many cancers are associated with aneuploidy, variations in the chromosome number, but it is uncertain whether this is a cause or an effect of cancer.