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Injury is physiological damage to the living tissue of any organism, whether in humans, in other animals, or in plants.

Injuries can be caused in many ways, including mechanically with penetration by sharp objects such as teeth or with blunt objects, by heat or cold, or by venoms and biotoxins. Injury prompts an inflammatory response in many taxa of animals; this prompts wound healing. In both plants and animals, substances are often released to help to occlude the wound, limiting loss of fluids and the entry of pathogens such as bacteria. Many organisms secrete antimicrobial chemicals which limit wound infection; in addition, animals have a variety of immune responses for the same purpose. Both plants and animals have regrowth mechanisms which may result in complete or partial healing over the injury. Cells too can repair damage to a certain degree.

Injury in animals is sometimes defined as mechanical damage to anatomical structure,^[1] but it has a wider connotation of physical damage with any cause, including drowning, burns, and poisoning.^[2] Such damage may result from attempted predation, territorial fights, falls, and abiotic factors.^[2]

Injury prompts an inflammatory response in animals of many different phyla;^[3] this prompts coagulation of the blood or body fluid,^[4] followed by wound healing, which may be rapid, as in the cnidaria.^[3] Arthropods are able to repair injuries to the cuticle that forms their exoskeleton to some extent.^[5]

Animals in several phyla, including annelids, arthropods, cnidaria, molluscs, nematodes, and vertebrates are able to produce antimicrobial peptides to fight off infection following an injury.^[1]

Injury in humans has been studied extensively for its importance in medicine. Much of medical practice, including emergency medicine and pain management, is dedicated to the treatment of injuries.^[6][7] The World Health Organization has developed a classification of injuries in humans by categories including mechanism, objects/substances producing injury, place of occurrence, activity when injured and the role of human intent.^[8] In addition to physical harm, injuries can cause psychological harm, including post-traumatic stress disorder.^[9]

In plants, injuries result from the eating of plant parts by herbivorous animals including insects and mammals,^[10] from damage to tissues by plant pathogens such as bacteria and fungi, which may gain entry after herbivore damage or in other ways,^[11] and from abiotic factors such as heat,^[12] freezing,^[13] flooding,^[14] lightning,^[15] and pollutants^[16] such as ozone.^[17] Plants respond to injury by signalling that damage has occurred,^[18] by secreting materials to seal off the damaged area,^[19] by producing antimicrobial chemicals,^[20][21] and in woody plants by regrowing over wounds.^[22][23][24]

Cell injury is a variety of changes of stress that a cell suffers due to external as well as internal environmental changes. Amongst other causes, this can be due to physical, chemical, infectious, biological, nutritional or immunological factors. Cell damage can be reversible or irreversible. Depending on the extent of injury, the cellular response may be adaptive and where possible, homeostasis is restored.^[25] Cell death occurs when the severity of the injury exceeds the cell's ability to repair itself.^[26] Cell death is relative to both the length of exposure to a harmful stimulus and the severity of the damage caused.^[25]