A bug in poker is a card that acts as a partial wild card. It can be used in any of the following ways:
- It can complete a flush by acting as the fifth card of the suit needed to make a flush. In so doing, the bug has a rank of "zero" (below a two) for purposes of comparing two flushes.
- It can complete a straight by acting as a card of the necessary rank to complete a five-card straight. It can act as a card of any rank, so if a player has four cards in sequence, the bug can act as the card on the high end of the sequence, forming a higher straight.
- It can complete a straight flush by acting as the fifth card needed.
- If not acting to complete a flush or straight, it acts as though it were an Ace. It can therefore be used to make "five aces" if the player holds the other four aces, though the rules about allowing five-of-a-kind may differ from casino to casino. If five-aces is allowed, it becomes the highest possible hand (higher than a Royal Flush).
Most commonly, one or both of the Jokers in a deck of cards act as a bug. This is commonly found in games of five-card draw (even when played online), but can also be seen in various other more obscure variants of poker. Bugs are not nearly as common as they once were in poker, but some games, casinos, and home games do still use them.
In some stud variants like Mexican Poker, a bug is played as stated above only if it is dealt as an upcard. If it is dealt as a downcard, it is considered to be a full wild card and can be played as any other card.