Each bullet appears as a small, dark-colored ball that can be loaded into and fired normally from a flintlock. However, each round contains a dose of Queen's own Mummy virus, which immediately infects those who are shot leaving behind a small skull-shaped mark on the victim's skin in the place they were hit.
Due to the contagious nature of the virus, the victim will spread it to others upon contact, thus allowing for a wider range of effect and an increased number of casualties. This means that you don't have to necessarily hit the intended target but can instead simply choose to shoot those around them and use the victims as living weapons, a strategy Daifugo attempted to use against Monkey D. Luffy who had proven too elusive, by infecting nearby prisoners instead.
An Excite Shot (疫災散弾,Ekisaito Shotto?, literally meaning "Plague Shot") is a cannonball-sized variation of the Excite Bullet which instead contains 200 doses worth of the Mummy virus, as indicated by the number written on it in light text. Babanuki attempted to use one against a crowd surrounding Luffy's group inside of the Prisoner Mine by shooting it out of his elephant's trunk. However, the effects remain unseen as Luffy tangled the trunk, causing the shot to explode in Babanuki's chest. In the VIZ manga, these are called Plague Shots.
The "Excite" part of the Excite Bullets' name is a pun based on the the words "ekisai" (疫災,"ekisai"? literally "plague") and "excite" (エキサイト,ekisaito?). The Viz translation attempts to retain this pun by calling them "Plague Rounds", which sounds like "playgrounds".