Mould

Gladys Street, try a weak solution of bleach and water to clean mould first, it may save you the expense of replacing the frame, also is there a window to the downstairs loo? as mould only grows in damp conditions, if you can leave your windows slightly cracked open to allow a good circulation of air that normally stops or greatly reduces the chance of mould coming back.

are you sure it is mould ??? we have had a problem with somesort of termite eating the architrave around our doors in our underbuild -- only the framework and not the doors !!!!
The affected area looked dark but when pressed it just caved in!!!!
Hope yours is just mould and not this as it has been an ansolute nightmare.