Can I just leave rotting floorboards?

[ACIDTECHNO]

Just lifted cracked shower tray to reveal wet and rotten floorboards (tray was fitted directly onto floorboards). It seems the edges have been leaking for quite some time. I'm going to fit a new tray but before i do...
The wood around the edges is black and fairly obviously rotten, a screwdriver can be pushed straight threw in some places. Assuming the new tray is fitted properly and doesn't leak, is it OK to leave these rotten boards down? Does rot spread? If it dries out will it just stay as it is and not get worse, or is it possible it would spread to joists (and eventually house collapse!)
Is there any treatment I can apply, or do I need a joiner to fit new boards?

Cheers.

 

[Alastairreid]

Have a look at this.

www.alsnetbiz.com/homeimprovement/woodrot.html

 

[chirpychippy]

You will need to replace the sodden floor boards, whilst they might dry out eventually the damage caused by the moisture will render them useless. Whilst you have the floor boards up check the joists, if the shower has been leaking for a considerable time then the ends of the joists could have perished. You don't need a joiner to sort this problem out but you will need a carpenter or a builder why don't you post us a pic of the joists and we can have a look for you.

 

[ACIDTECHNO]

Thanks for that. Very interesting link. I'll take some pics tomorrow, but it won't be the joists, just the boards. I think I'll leave it for a friendly joiner I know.
I just hope the tiled floor doesn't have to come up (corner where shower is isn't tiled, tray was on floorboards).
There's a joist right under the wall that has had the worst leaking, and has the worst rot, so I just hope joist is OK.