A cloud is an object in Dr. Mario World. They are introduced in World 5. When the stage begins, the clouds also start out transparent but will immediately cover the objects they are covering. On occasion, the clouds will momentarily become transparent so as to reveal the objects that they are covering. Besides being covered by clouds, the objects behind them behave normally. For example, a virus hidden behind a cloud can still react to being matched by a capsule, and barriers toggle on or off when a capsule lands without eliminating anything. Some clouds do not contain any objects, which means that the cloud is solid and will not become transparent except during the beginning of the stage. Clouds will disappear when the objects behind them are eliminated, or in the case of a solid cloud, when a capsule match is made next to it or when a skill or item eliminates it. Dr. Toadette and Dr. Lakitu's versus mode skills hide the viruses on the opponent's playing field inside clouds, where the former can hide up to 20 viruses (10 prior to season 7) in them while the latter can hide up to 9 viruses (7 prior to season 7) in them.