No homes are almost sterile. Leaving aside the question of what connection cancer might have to the immune system (probably none), there are germs everywhere in the air and on every surface and more constantly come in because houses are not airtight even if you never step outside.
My parents were in India in the 50s. There were many Americans there looking for business in the newly independent State.
The English mostly manage pretty well in the heat, flies and generally unhygienic conditions prevailing. The Americans did not. My father always maintained that it was because the Americans had a lower immunity due to their high standard of hygiene, frequent showers, and zealous attempts to avoid germs.
Not to be a killjoy, but remember cancer is not one disease. It is a collection of conditions that manifest themselves in variously related ways. Yes you can ‘contract’ cancer from exposure to substances such as plutonium or asbestos, but you can also be genetically predisposed to develop it.