Hi, LinCan.
I understand being scared - I sure had my moments before surgery! There are just a lot of things out of your control in this process, and that's scary. But knee replacement is a pretty mature surgical procedure, and I can tell you that my care team pretty much thought of everything and planned for every contingency.
For example, post surgical infection is rare, (occurs in less than 1% of patients), but my care team took a number of different precautions to avoid it. I had special soap to wash with for several days before surgery, they had stringent draping and hand washing procedures at the hospital, I got prophylactic antibiotics with the surgery, repeated inspections and cleaning after, etc. I think it's great to be aware of the possibility and follow good procedures to avoid this and other complications, but the liklihood is pretty low.
Take it easy, and best of luck to you,
La