A virus is a lifeform, as is a fungus. Both have "a life of their own."
Cancer's a misfire in the body's immune system. We don't yet know all the possible triggers,
but the answer is VERY complicated because there's lots of risk factors, including some genetic ones
and exposure ones (it can run in families, exposure to a known carcinogen, especially over time, can lead to it).
However, we don't have ALL the answers on it now, we may never have all the answers to it.
I'd buy that devils could inspire unhealthy behaviours, and those unhealthy behaviours would be risk factors
to cause cancer. There's no need to make up an entire system of "and devils can affect biology directly,
but only under certain super-specific instances", which is what would be needed to try to accomodate the
twi doctrine in any consistent fashion.
In short, vpw made lots of stupid claims which were picked up by others. This was one of them. There's no
evidence of any kind to support this one that isn't easy to dismiss.