Why were some bacteria and viruses were radioactive at the end of the Hershey chase experiment?

Radioactive isotopes got scattered in the experiment which contained radion, however, the bacteria didn't have a high amount of radiation. You even eat radiation every day. For Example, a banana has approximately 1/1,000,000 sieverts. in order for that radiation to kill you, you would need the radiation of 2 sieverts, or 2 trillion bananas.