The NHS puts lots of effort into planning for a major incident - whether it is a terrorist attack, a cyber-attack, an outbreak of infectious disease or simply a major power cut.
But what the past few months have shown is that the dedication and goodwill of staff play a vital role.
After both the Westminster Bridge and Manchester concert attacks, hospitals ended up turning away doctors, nurses and other staff who were volunteering to come in.
The actions of Dr Malik Ramadhan, divisional director of emergency care and trauma at the Royal London Hospital, where 12 of the London Bridge and Borough Market victims were taken, are a perfect illustration of this.
Dr Ramadhan had finished his shift and was cycling home over Tower Bridge at the time of Saturday night's attacks.
"I was completely oblivious," he says, "and as I got to the Old Kent Road a large number of police vehicles came whizzing past, more than I've seen before, and I thought that's a bit unusual.
"Given what's been happening, I thought I had better go back to work."
Dr Ramadhan got back to the hospital and was told it had been put on a major incident alert and to expect multiple casualties.
"We don't get told specifics," he says.
"We get told something really bad has happened, and we have a plan to prepare for something really bad."
The team started clearing beds, contacting on-call staff and messaging colleagues to see who might be able to come in and help.
"By the time patients arrived, we had fully staffed resuscitation bays to receive each of the patients," Dr Ramadhan says.
"The 12 were all very badly injured. The people who were stabbed had been stabbed with the clear intent to kill."