If you tear your intestine then yes, you probably will get a very nasty infection (if this happens don't wait to see what happens - go to the doctor).
If you're careful and reasonably clean (I wash my toys after use with soap and water, and I'm convinced that's "reasonably clean") there shouldn't be a problem from them.
If you bruse your prostate or other internal organs you can have problems.
AFAIK there's no link between anal play and cancer, and there isn't any likely way that anal play will cause the genetic damage necessary to cause cancer. I would be careful about the chemicals on things I put up there, though - stick to silicone, glass, metal, or hard plastic and safe lubes so you don't get any carcinogens up there.