Due to our long lifespan, humans are relatively vulnerable to radiation, radioactive materials, and other bioaccumulative poisons. A fish might not accumulate enough mercury to kill itself over its lifetime, but when you eat one every day it all adds up.
This was why the disaster was so bad for so many farmers across Europe: https://www.bbc.com/news/uk-wales-36112372 ; the caesium is not enough to kill a sheep, which has a life of one or two years before slaughter, but should not be consumed by humans.
https://www.npr.org/sections/thesalt/2014/06/03/318241738/ho...
Of-course there were radionuclides in our bodies even before the first nuclear test in 1945. For example Potassium-40 or Carbon-14. The presence of Carbon-14 in organic matter is the basis of the radiocarbon dating method to date archaeological, geological and hydrogeological samples.
The big question is how much radionuclides is safe and how much radionuclides is a health risk.
https://en.wikipedia.org/wiki/Dose%E2%80%93response_relation...
For a long time there was a serious debate over whether wild animals actually experienced aging or not, because they’d never live long enough to get noticeably aged.
There are dogs roaming around the Buryakovka nuclear waste storage facility. About ~10 years ago I have been told that their average lifespan was in a ballpark of three years. Make what you will from it.
OTOH Przewalski's horses are just thriving in the Zone!