That's because everything DOES
Cancer is caused by a mutation within a cells apoptosis process.
A gene for the process is changed, and the process is thus stopped.
Now usually, tumorous cells are eliminated, but sometimes, due to effects from the environment, said elimination fails and the process continues. The cells divide and never die, creating a tumor
If the tumor is malignant, it can metastasize and spread along the blood of lymph, which in turn will contaminate the entire body and lead to death
There's two factors here:
The genetic predisposition of an individual to both develop cancerous cells, and to be influenced in such a way by the environment that the genetic process of eliminating these cells fails, and the effect of the environment on the mutant of said genes
There are many things that can cause this effect. Heck just about anything can, but it needs to be in sufficient quantity for each substance. Things like nicotine require less quantity.
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