So would an organism that uses XNA not be affected by any bacteria, viruses, etc., that would affect a normal, DNA using organism?
Viruses yes; bacteria no. The "immunity" to these different things depends on how the pathogen utilizes the host organism.
Bacteria have their own cellular machinery to grow and reproduce and the host they infect is simply a source of readily-available nutrients. If they are DNA-based bacteria infecting an XNA-based host the fats and sugars and other nutrients are all still there to be taken regardless of the XNA/DNA mismatch so the bacteria can still eat and thrive.
The same is true for protozoan and or fungal infections.
(Some details will matter though. For example, humans don't digest cellulose so we can't just live off of wood/grass/etc. and similarly couldn't live off a XNA plant that stores its energy as cellulose. If that XNA plant stores it's energy as everyday fructose and glucose, however, it's fair game. The same concept applies to whatever specifically it is that the bacteria/fungi/protozoans are digesting.)
Viruses are different because they hijack the host's cellular machinery in order to replicate; if the host's cellular machinery only reads/writes XNA then a DNA-based viral payload is not going to be able to be read or copied even if managed to sneak in.
Prions would form another category. Unlike viruses they do not hijack the DNA/XNA machinery to replicate, but they do still require that the organism in question have a matching protein structure that can cause a misfolding cascade. It doesn't really matter whether the protein in question was synthesized from DNA or XNA instructions, but the likelihood of convergent evolution creating two proteins analogous enough to one another that they can prion-misfold eachother is vanishingly unlikely.