Injury is damage or harm caused to the structure or function of the body caused by an outside agent or force, which may be physical or chemical.

Types of Injury

1. Burns are injuries caused by excess heat or sometimes cold. In medicine, a burn is a type of injury to the skin caused by heat, electricity, chemicals, or radiation.

Immediate first aid for burns consists of immersing the injured area in cool clean water to cool the injured tissues.

There are in modern terminology three degrees of burns.

First-degree burns are usually limited to redness and pain at the site

Second-degree burns additionally have blistering of the skin

Third-degree burns additionally have charring of the skin or eschar formation. An eschar is a scab that has separated from the unaffected part of the body. These are painless because at this stage the nerve endings are burnt away.

Burns that injure the tissues underlying the skin, such as the muscles or bones, are sometimes characterized as "fourth-degree burns". Serious burns, especially if they cover large areas of the body, can cause death; any hint of burn injury to the lungs, for example through smoke inhalation, is a medical emergency.

Chemical burns are usually caused by chemical compounds, such as sodium hydroxide,  silver nitrate, and more serious compounds. Note that most chemicals that can cause moderate to severe chemical burns are strong acids or bases. Nitric acid is possibly one of the worst burn-causing chemicals, as an oxidizer. Most chemicals that can cause moderate to severe chemical burns are called caustic.

Electrical burns are generally symptoms of electrocution, being struck by lightning, being defibrillated or cardioverted without conductive gel, etc. Survival of severe burn injuries is markedly improved if the patient is treated in a specialized burn center rather than a hospital.

2. Fractures are injuries to bones. A fractured bone in a living person is typically treated by immobilization with a plaster or fiberglass cast, and in some cases surgical nails, screws, plates and wires to supplement a cast. Occasionally smaller bones, such as toes, may be treated without the cast, by buddy wrapping them, which serves a similar function to making a cast. By allowing only limited movement, fixation helps preserve anatomical alignment while enabling callus formation, towards the target of achieving  union. Operative methods of treating fractures have their own risks and benefits, but usually surgery is done only if the risk of conservative treatment outweighs the potential benefit of an operation.

In medicine, fractures are classified as closed or open (compound) and simple or comminuted. Closed fractures are fractures where the skin is intact and there is no bone poking out while open (compound) fractures involve wounds that expose bone. Open injuries carry an elevated risk of infection and at minimum require antibiotic treatment, and in fact many require urgent surgical treatment. The only method of accurately diagnosing a closed fracture is by having an X-Ray photograph taken of the injury. Simple fractures are fractures that occur along one line, splitting the bone into two pieces while comminuted fractures involve the bone splitting into multiple pieces. A simple, closed fracture is much easier to treat and has a much better prognosis than an open, comminuted fracture. Other considerations in fracture care are displacement (fracture gap) and angulation. If angulation or displacement are large, reduction (manipulation) of the bone may be required and, in adults, frequently requires surgical care. These injuries may take longer to heal than injuries without displacement or angulation. In children, whose bones are still developing, there is the risk of a greenstick fracture. This type of fracture occurs because the bone is not as brittle as it would be in an adult, and thus does not completely fracture, but exhibits bowing without complete disruption of the cortex. Plastic deformation of the bone is also possible in children, a situation in which the bone bends but does not break. These injuries may require an osteotomy (bone cut) to realign the bone if it is fixed and cannot be realigned by closed methods.

3. Wound: cuts and grazes are injuries to the skin, that can cause bleeding. A wound is a physical trauma where the skin is torn, cut or punctured. Exposed to air, there is not only a significant chance of death due to loss of  blood, but increased chance of bacteria entering the wound or an infection.

A person suffering from a significant wound may go into shock, and need serious medical attention. Wounds should be kept clean, and closed if possible until professional help is available.

Before any medical or paramedical evaluation, a wound is considered as minor when:

• it is superficial;
• it is away from natural orifices;
• there is only a minor bleeding;
• it was not caused by a tool or an animal.

Any other wound should be considered as severe. If there is any doubt, a wound should be considered as severe. "Severe" does not necessarily means that it endangers life, but it must at least be seen by a physician.

Types of wound

Wounds can be classified into a number of different types, according to the object that caused the wound. The types of wound are:

Incisions - caused by a clean, sharp-edged object such as a knife or a razor.

Lacerations - rough, irregular wounds caused by crushing or ripping forces.

Abrasions (grazes) - a superficial wound in which the topmost layers of the skin are scraped off, often caused by a sliding fall onto a rough surface.

Puncture wounds - caused by an object puncturing the skin, such as a nail or needle.

Stab wounds - caused by an object such as a knife entering the body. Gunshot wounds - caused by a bullet or similar projectile driving into or through the body. There may be two wounds, one at the site of entry and one at the site of exit.

All stab wounds and gunshot wounds should be considered major wounds.

4. A bruise is a haemorrhage under the skin caused by contusion. A bruise or contusion or ecchymosis is a kind of injury, usually caused by blunt impact, in which the  capillaries are damaged, allowing blood to seep into the surrounding tissue. Normally minor but painful, bruises can be serious, leading to hematoma, or can be associated with serious injuries, including fractures and internal bleeding. Minor bruises are easily recognized by their characteristic blue or purple color in the days following the injury.

The word "bruise" is also used for fruit - a fruit is bruised when an impact breaks the internal water-containing structures, leading to a soft spot. More generally, "bruise" is used metaphorically to mean any minor injury: one's ego might be bruised if one's painting was not included in a gallery showing, for example. The implication is that the injury is minor but painful, but will recover on its own.

The presence of bruises may be seen in patients with platelet or coagulation disorders. Unexplained bruising may be a warning sign of child abuse or serious medical problems, such as leukemia and meningoccocal infection. Anybody with unexplained bruising needs urgent examination by a doctor.