the sun does cause this but it is now understudy that it is because we don't get the full spectrum of the suns rays. apparently uva (eg some solar beds & sun through glass, glass blocks uvb) is the spectrum that causes the damage to our skin but when recieving uvb & the full aray of the suns effect we are protected. this means that within poluted cities & atmospheres we are supposedly more subject to skin cancers etc as the polution blocks the uvb from reaching us.
One way we can help protect & even cure from cancer situations is via alkalizing ourselves this can be done through diet & is used in alternative treatments for cancer with a reported high rate or success.
You are right about a deficiency within our immune system, it is our deficient immune system or acid state in our bodies that causes all illness not so much as the bacteria or dirt that we are subjected to. the manipulation of our natural foods over the years has been bought about by greed & the love of money, this manipulation has caused the food to become deficient of different nutrients or changed the nutrient to a less absorbable structure which then renders us deficient through not being able to absorb as much as would be originally absorbed. I am willing to share what i have learned with anyone who is willing to learn these things.