Post-traumatic stress disorder (PTSD) is an anxiety disorder that can develop after being involved in, or witnessing, traumatic events.
A wide range of experiences can cause it, from being in a car crash, a traumatic childbirth, losing someone close to you in upsetting circumstances, being raped or assaulted, to surviving a terrorist attack or being in a war.
Ms Rachel Boyd of British mental health charity Mind said: "If you experience trauma, it's natural that it has an impact on your life."
If you are affected for a long period or have intense symptoms - such as anxiety, reliving the event or nightmares - you could have PTSD, she said. And it can take years for symptoms to emerge.
PTSD can cause disturbing flashbacks and recurring nightmares.
You may feel "hyper-alert" - unable to relax your guard, sleep or unwind. Or you may go to unusual lengths to avoid thinking about the trauma by staying away from places, people or situations that remind you of the event.
PTSD may also cause physical symptoms associated with anxiety, such as palpitations, diarrhoea and headaches.
PTSD is not a sign of weakness; anyone can be affected.
But if you have experienced emotional, physical or sexual abuse and neglect as a child, you are more vulnerable because vital relationships of trust may not have developed normally.
This early damage may affect all relationships in future.
Your personal resilience is a complex mix of genetics, temperament, hormones, mental and physical illness, support systems and previous experiences. But even the most resilient person can be knocked for six by PTSD.
Having a good support system and looking after your physical and mental well-being make sense, but may not be within your control.
Your workplace should offer training, support and risk management.
Trauma Risk Management is a peer support system developed by the British army and widely used in workplaces where personnel are exposed to a high risk of trauma.
START TREATMENT EARLY
People with mild symptoms of PTSD may get better within a month without treatment. But if the initial response to the trauma is severe, treatment should start sooner rather than later, otherwise it becomes harder to treat.
Everyone who has experienced trauma should be given practical support in an emphatic manner by health and social care-workers, but offering everyone one-to-one debriefing that focuses on the event is not recommended and may do more harm than good.
The recommended treatment depends on the kind of trauma you have experienced and how the symptoms of PTSD affect your life, said Ms Boyd. There is no one-size-fits-all approach, but recommended therapies include trauma-focused cognitive behavioural therapy (CBT), eye-movement desensitisation and reprocessing treatment, which focuses on helping your brain to process the trauma.
Medication can be used alongside other treatments. Some people find exercise, mindfulness and art therapies helpful.
Dialectical behaviour therapy (DBT) is a modified form of CBT developed by the US psychologist Marsha Linehan.
CBT focuses on helping to change unhelpful thinking and behaviours, but DBT also focuses on accepting who you are and puts a lot of emphasis on your relationship with the therapist.
The goal is emotional self-regulation, which means you can keep your emotional responses tolerable and within your control.