What are cancer survivors deferred from donating blood?

So far as we know, cancer isn’t contagious. If any stray cancer cells got into someone’s donation, wouldn’t the recipient’s body just kill 'em? Or is this a precaution against something else?

According to this:

Apparently, it’s not a sweeping ban on donations for all cancer survivors, and the restrictions on who can and cannot donate can differ between organizations.

I was told that one Red Cross restriction was if you ever had chemotherapy. I don’t know the reason for sure. It may be a “better safe than sorry” thing, since we don’t know all the underlying causes of cancer.

Blood donation organization (like the Red Cross) generally do have a better safe than sorry rule for a lot of things, on the off chance something gets through their testing. Personally, I think their donation rules go too far (the fact that they might reject you just because your gay. I’m not sure of the statistics, but I’m pretty sure the rate of AIDS and STD’s in homosexuals is probably pretty close to that of heterosexuals now. It’s not the 1970’s anymore.) But I digress.

But yeah, the organizations don’t want to be sued or kill off the people their blood is designed to save, so if there’s a small chance it could hurt someone, they’ll probably reject the blood.