Soft tissue or anything that is not "dense" will not show up on an xray clearly. Radiation mostly goes through soft tissue and thus shows dark shades on the film. Only dense tissue (such as bones) blocks the xrays which shows up as light on the xray film. It is this contrast that visually seperates dense from soft tissue on the film. Since parasites are all soft tissue, they do NOT show up on an xray. Now let's say a gerbil got inside you (for some odd reason), that *will* show up due to the fact that gerbils have spinal columns (dense tissue).

There are some devices that WILL detect parasites using sophisticated bio resonance methods.