Don't they see too much?

Don't they see too much on a scan, everyone body has some illness?

A scan is very accurate, indeed in many cases something can been seen, but is of no meaning as such. This is called "false disturbance" or "false positive", an example is e.g. scar tissue. It is the task of the medical specialist to separate the important issues from the trivial. They have to give a good interpretation of the images. The doctors ensure that you do not have to worry unnecessarily. In some cases, further examination is needed for clarity.