Certain detrimental spells have 'counters'; if you remove all of the counters, the spell is eliminated.
so my understanding is.
If say you have the Tashan spell effect on you. Your poison counter will be 1.
(Tash may not be a good example I don't really know if cure poison will work on it)
http://wiki.project1999.com/Tashan
You can use Cure Poison to remove it (-1 poison counter)
http://wiki.project1999.com/Cure_Poison
But if you had say Tainted Breath cast on you (+5 poison counter)
http://wiki.project1999.com/Tainted_Breath
You'd have to cast Cure Poison 5 times.
OR
1 cast of Counteract Poison(-8 poison counter) to remove Tainted Breath(+5 Poison Counter).
http://wiki.project1999.com/Counteract_Poison
I do not think you can ever go below 0 counter.
The mechanic is the same for the Disease counter too.