Effect of an isolation transformer on leakage current
The point of a isolation transformer is to break any common mode connection to the rest of the world.  This therefore allows one connection to the rest of the world that won't draw any current.  Usually you try to not have any such connection, so that a accidental connection (you touching a live wire with one hand while grabbing a metal drain pipe with the other, for example) is the first connection and therefore safe.

A common mistake people make when using a isolation tranformer is to forget that connecting a oscilloscope usually means connection some point to real earth ground.  That by itself is OK since it is only a single connection.  However, now the circuit is no longer isolated and significant current can flow between some part of the circuit and a metal pipe, for example.