It’s a reductio ad absurdism, I’m not saying everyone has potential cancer, I’m saying that sometimes protecting your health isn’t convenient or fun. The idea is that it is temporary.
Cancer does not spread rapidly through air.
Also, chemotherapy is done after you get cancer. It is not a preventative measure.
It is preventative though, it prevents the cancer from progressing and metastasizing.
It is not preventative in the sense that you can't ask millions of people to get chemotherapy just in case.