Those recovering from a traumatic event - such as a school shooting or a major crash - are at risk of severe psychological harm.
The main risk is that they will suffer post-traumatic stress disorder, which can lead to painful memories striking the victims at any time.
What is post-traumatic stress disorder?
The syndrome is a disturbing psychological condition where people relive painful memories against their will.
This leads to feelings of isolation and a sense of losing control - patients sometimes turn to alcohol or other drugs as they attempt to get rid of the memories.
It is also known as shell shock because it first came to prominence when soldiers in the First World War were afflicted by memories of the horrors they saw in the trenches.
Dr Matthew Freidman, of the US Department for Veteran Affairs, says the experience is like living through the original event a second time.
"It is like a psychic time machine," he says.
Who gets it and what causes it?
Anyone who has witnessed acts of violence - either on themselves or others - is particularly prone to the syndrome.
Severely disturbing event such as being raped or robbed, or exposure to a highly stressful environment, such as a war zone, are known to trigger post-traumatic stress disorder.
However, an individual's personality is also responsible for determining a person's susceptibility to the syndrome.
The memories can be triggered by the slightest detail reminiscent of the original traumatic event.
Ms Margaret Searle is a clinical psychologist who for more than ten years has helped military personnel overcome the condition.
She says no-one fully understands the mind's workings but latest research suggests memory works by connections rather than being stored in neat units.
This makes it easier for unpleasant memories to occur unsummoned.
She says: "The memory for an event is distributed among various sites and the process of remembering involves the creation of a variety of connections."
A memory can be called up by various means, she added. "It can be called up by a particular sound, it can be called up by a particular sight, or a particular smell."
Memories which have many of these sensory elements - as is often the case with traumatic memories - can suddenly be called into a person's mind unrequested.
How does the disorder affect children?
The Global Information Networks In Education project says children of different ages will respond differently to traumatic events.
It says symptoms adults can spot include:
- Emotional detachment
- Feelings of guilt
- Excessive vigilance
They can also have difficulties concentrating and sleeping, or may be plagued by nightmares.
What is the treatment?
The psychiatric journal Harvard Mental Health Letter outlines a typical treatment pattern for post-traumatic stress disorder.
It says: "Most psychotherapists who treat PTSD try to provide an opportunity for the patient to feel safe in confronting the traumatic event and understanding its connection with their symptoms.
"Patients are helped to think about the trauma without intrusive reliving and exercise self-control without avoidance and emotional numbing."
It says seeking help as soon after the traumatic event as possible is essential. Patients can then receive "emotional first aid" in the form of group or individual counselling.
"They will be encouraged to review the experience, tell their stories, and express their feelings.
"They will learn that their responses are normal and be advised to resume everyday activities as soon as possible."
It says such early intervention can prevent short-term trauma having an impact in the long term.
However, it also says that some patients will be reluctant to discuss their experiences, and, because of the cause of the disorder, perhaps with good reason.
The challenge for the therapist, it says, is to remain calm while listening to horrifying stories.
"Therapists have to avoid both over-commitment and detachment. They should not identify themselves with the patient or see themselves as saviours," it says.
It notes that some patients find it easier to discuss their memories under hypnosis.
Other therapists, it says, feel it is not so important to relive and deal with the traumatic memory itself as to cope with present symptoms and problems.
They use the same techniques that are used to help people overcome phobias to help people overcome fear of things that evoke the trauma.
In severe cases drugs such as Prozac can be used.
Is there help?
Ms Searle says the syndrome requires professional attention.
She advises: "If people are experiencing problems then a good route to try and access help is via the NHS, via their GP and a referral to psychiatric and psychological services.
"There are tremendous problems about resources in the NHS, but there are also tremendous problems for people experiencing these kind of difficulties."
However, David Bennett, director of the UK Trauma Centre, warned the average wait to see a specialist - if one was available at all - varied from nine to 18 months.
He told BBC News Online: "At present for any patient to receive treatment for PTSD it is near impossible."
Mr Bennett said PTSD was often misdiagnosed as depression or anxiety, or even ME.
"It is a sad state of affairs. Many who profess to be so called experts, counsellors and psychologists do not appreciate the seriousness of this illness," he said.
"It sometimes breaks my heart when I deal with these individuals who are now broken human beings, even unable to leave their homes for treatment."
This page contains basic information. If you are concerned about your health, you should consult a doctor.