The granite used to build houses and grand building is radioactive.  In fact, you get more radiation from living in a stone house than from the peak leakage from 3 mile island.

Radiation is a "dose makes the poison" thing.  Too much and you get nasty burns and probably cancer.  Too little, and you get cancer.  When the dental industry rolled out CCD cameras for dental X-ray, they required an order of magnitude less x-ray exposure than film.  Shockingly, cancer rates went up with the new equipment.  It turns out that such tiny exposures are worse than the exposures required for film.   Problem easily solved - a little research to find the optimum dose, and cancer rates are down as desired.

A while back there was the BitRot Windows virus which would flip one bit chosen at random on the disk.  Users would not notice anything amiss - enhancing transmission.  After enough bits were flipped, however, their data would be catastrophically ruined.   Modern filesystems like btrfs checksum everything - and catch this kind of thing, but have more overhead.  Similarly, wildlife at Chernobyl is thriving.  They have shifted into high radiation mode, which takes more energy.  But then there are few humans around to bother them.  (There are videos of motorcycle tours of Chernobyl that are fun to watch - they carefully monitor their dose.)

People used to pay to sit in a uranium cave.  A little exposure causes causes some temporary inflammation, which cured certain kinds of disease.