At first, a common cold could be feigned to cover it, but mortals quickly develop a type of leprosy which covers their body in lesions. This sickness can not be stopped unless they return back through the wells to whence they came.
Many do not make it back to the wells and end up dying on their journey. Because of this sickness, it takes a certain period of time to recover to a native state, and even people in advanced stages can perish after returning home.
The fundamental "frequency" which physical matter exists at the sub-atomic level is extremely unique to the universe that Orr exists within. You can think about our universe as being a "perpendicular universe" as opposed to a "parallel" one.
Nothing can maintain a permanent residence from or in our universe after travelling to or from another, respectively, i.e. it works both ways. Matter breaks down to an exponential rate as it approaches 60 days within the universe foreign to it, which in Orrian time is about a month.
This desynchronization of natural "frequency" with the surrounding universe can be corrected by returning to the correct universe. However, the recovery, or resynchronization, is not instant, often taking as long as the desync time to recover.