How can I reduce interference issues when installing Cat 5e and 6 cable?

Category 5e (which replaced category 5) can be run 100 meters depending on the application that will run over the cable. However, in order to test category 5e specifications, it must be properly installed. This includes things like bend radius, pull tension, termination and other installation factors. Most companies are installing category 6 as category 5e is near the end of its lifecycle for a standard. Category 6 cabling is a much heartier system and is generally the minimum recommended by all the newer standards.

There are things that can interfere, in particular in a hospital, particularly around the teleradioloy or radiology...

areas. Many hospitals opt for shielded cabling systems (still generally Category 6), however some still use the unshielded variety. But pay particular attention to where the cable paths are run. This may also be true around dialysis machines and other machines that require higher voltages. No cabling system will work properly or support your applications if it was not installed properly.

As for your plenum question, that depends on whether the return vents are ducted or not. If they are not (which is usually the case) then yes, the cable must be plenum rated. There is also language in the new national electrical code that states that it must be limited combustible cable or those areas must be have sprinklers. Your local inspector can provide you with guidance in this area.