Proliferate

Proliferate is a keyword action introduced in Scars of Mirrodin. When a player proliferates, they choose any number of players and/or permanents that have counters on them, then puts another of each kind of counter that are already there on these players or permanents.

History
Proliferate was introduced in Scars of Mirrodin and took advantage of the new infect mechanic, appearing only on blue and artifact cards. In Mirrodin Besieged, it also showed up on one card each in black and green. In New Phyrexia it also appeared on one card in red.

Proliferate was considered for Kaladesh and Aether Revolt to take advantage of mechanics such as energy counters and Fabricate, and for the Simic guild mechanic for Ravnica Allegiance, to go along with the guild's +1/+1 counter theme, but did not make it in.

Proliferate's next appearance was in War of the Spark, taking advantage of both the number of planeswalkers in this set and the Amass mechanic. This set doubled the total number of cards with proliferate and included cards in white, blue and green. For this set, the rules for proliferate were changed; instead of adding only one counter to each permanent or player, each proliferated permanent or player now gets one counter of each kind they already have.

As with many other mechanics, proliferate next appeared in Modern Horizons. It was featured on one black card and colorless card.

Description
For example, a player proliferates and chooses an opponent with a Poison counter, a Planeswalker card with a loyalty counter, a creature with a -1/-1 counter and an artifact with a charge counter on it. Then that opponent gets another poison counter, the planeswalker gets another loyalty counter, the creature now has two -1/-1 counters and the artifact two charge counters on it. Note that proliferate does not target, so permanents with shroud or hexproof can be chosen.

Thematically, the proliferate mechanic is tied in with the Phyrexia faction in Scars of Mirrodin and its related Infect mechanic. As such, each of the cards that have been printed with proliferate are marked with the symbol of the Phyrexia faction. Only featured on blue and artifact cards in Scars of Mirrodin, it has spread to black and green in Mirrodin Besieged, illustrating the block theme/storyline of the Phyrexian infestation taking over Mirrodin.

In War of the Spark, it instead represents "the influence of the planeswakers": the heroes in blue, white and green - in opposition of Bolas in Grixis - building up creatures with +1/+1 counters and the loyalty counters of the many Planeswalkers, and additionally synergizing with the Amass tokens.

Rulings

 * You can choose any player that has a counter, including yourself.
 * You can choose any permanent that has a counter, including ones controlled by opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them, such as suspended cards or a Lightning Storm on the stack.
 * You don't have to choose every permanent or player that has a counter, only the ones you want to add another counter to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.
 * If a permanent chosen this way has multiple kinds of counters on it, a new counter for each kind is put on that permanent.
 * Players can respond to the spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.
 * When you proliferate in a Two-Headed Giant game, you may give one poison counter to a team that already has one.

Reminder text
The latest reminder text for proliferate reads "Choose any number of permanents and/or players, then give each another counter of each kind already there."

Previously it read "You choose any number of permanents and/or players with counters on them, then give each another counter of a kind already there". This means that older versions of proliferate only increased one type of counter while the updated version increases each type of counter.

Development
Mark Rosewater originally developed a mechanism for creatures to amplify infect by adding -1/-1 counters to those already on other creatures. This mechanism furthered the metaphor of a Phyrexian disease spreading. Eventually, it grew to affect all counters.

Trivia

 * Proliferate was featured as rules card 3 of 5 in the Scars of Mirrodin set, 5 of 5 in the Mirrodin Besieged set, and 4 of 4 in the New Phyrexia set.