There are 5 Special Conditions all together.
2 of them use Markers to indicate that they are in effect.
3 of them use a turn of the Pokemon card to do the same.
You can place a marker for Poison and/or Burned without interrupting any other Special Condition.
However, you can only turn a Pokemon one direction at a time.
Therefore, a Pokemon can only be one of Confused, Asleep, or Paralyzed. Once you turn the card to indicate a different one of these three, the previous one is removed.
So, your Pokemon can be Burned, Poisoned, and one of the remaining three at the same time.