Injuries typically are received by falling in combat, and can affect the player character as well as their companions. They reduce one or more stats until treated, which can be done in various ways. Injuries can stack.
Dragon Age: Origins
Characters, including The Warden, that fall in combat receive a debilitating and persistent injury that lowers an attribute, another stat or the healing rate until treated. These injuries can stack, though a character cannot have more than one of each type.
Taking the wrong item from the chests in the quest Jammer's Stash will also inflict an injury.
To remove injuries, either return to your party camp, benefit from a Spirit Healer's Cleansing Aura (which does not heal its caster's injuries), or apply an injury kit. As with poultices, each kit has a certain level of potency that determines how much damage it is able to repair: Lesser Injury Kits remove a single injury, Injury Kits remove three injuries, Greater Injury Kits remove all injuries.
Dragon Age II
In Dragon Age II there is only one, but stacking injury type, reducing the character's maximum health until treated. These reductions in maximum health also tend to make the character less resilient in battle, suffering knockback from basic attacks where they were not a problem in earlier encounters with the same enemies. That combined with the obvious lessening of max health makes it easier for them to fall in battle again, thus creating a vicious cycle in which one injury facilitates the next.
To treat injuries, a character must consume an Injury Kit or an Elfroot Potion from the Potions menu, or the player must return home or blow one of the party gathering horns stationed around Kirkwall. Returning to Hawke's home tends to be the most cost-effective way of treating injuries, as the unused kits can be sold for some extra silver or immediately stored in the storage chest until a long quest that does not afford the opportunity to return home (such as The Deep Roads Expedition). (Hawke's home also tends to load quicker and clicking the door gathers the party and returns to the map in a single action, which saves time compared to other locations.)
The maximum number of injuries a character can suffer at the same time depends on the difficulty level. As Nightmare difficulty stops yielding Injury Kits after obtaining four of them, it is important to treat injuries as soon as they appear, at the very least in the hopes that enemies will drop more, if not to prevent injuries from compounding. In some situations where enemies remain but the party is far enough away, the game may trigger the fallen characters to revive, with only 1 health that does not heal any further due to the continued presence of enemies, then suffer a bleed effect that strips that final point, causing them to immediately KO again, all in the span of one second. This continues to pile injuries on the character to the point that they are rendered useless in battle, even if the player survives the current encounter, unless they go home and heal every injury.
Lastly, the Spirit Healer passive ability Second Chance makes party members immune to injuries so long as the healer is present. As the party's only potential Spirit Healer, Hawke, is always present, this effectively eliminates the injury mechanic for the rest of the game. (Bethany becomes a Force Mage later in the game, and Anders does not have a similar passive ability in his Vengeance tree, despite being the designated healer. Thus, this perk is a potential luxury of mage Hawkes only.)
Dragon Age: Inquisition
There are no injuries in Dragon Age: Inquisition, as the mechanic has been removed entirely. In exchange, most forms of healing magic have also been removed, with only a handful of abilities (such as a Knight-Enchanter's Resurgence Focus ability) restoring health. All practical healing is done by a communal stock of 8 Healing Potions (increasable to 12 with the More Healing Potions Inquisition perk), which can be replenished at any Inquisition camp, Haven, or Skyhold, or by opening a Supply Cache, and their effects can be improved with research and the right crafting materials. Alternatively, the player can craft specialty items like Regeneration Potions and Healing Mist grenades, but they require materials just to craft and restock as well as research improvements, or craft equipment with the "Heal on Kill" perk. In addition, allies can be revived with a button prompt over their KO'd body (hold the action button for a few seconds) rather than having to do without them the rest of the fight or use a spell like Revival (which still exists in this game). However, the button prompt only revives companions with a certain amount of health, decreasing at every higher stage of difficulty. On Nightmare difficulty it only provides a tiny sliver, making the Revival spell more useful and safer.
In short, while injuries have been removed, health management as a whole has changed in the game, and addressing these concerns requires a similar tact as treating injuries in previous games.