Super-spreading, where individual patients pass on an infection to large numbers of people, is a feature of nearly every outbreak.
It is not their fault but can have a significant impact on how diseases spread.
There are reports of super-spreading during the new coronavirus outbreak, which has centred on Wuhan, in China.
A British man who had been in Singapore has been linked to four cases in the UK, five in France and possibly one in Majorca.
Coronavirus claims 97 lives in one day
Four more people diagnosed in UK
How worried should we be?
What it does to the body
What is a super-spreader?
It is a bit of a vague term, with no strict scientific definition.
But it is when a patient infects significantly more people than usual.
On average, each person infected with the new coronavirus is passing it on to between two and three other people.
But this is only an average; some people will pass it on to nobody while others pass their infection on to far more.
How big can a super-spreading event be?
Massive – and they can have a huge effect on an outbreak.
In 2015, a super-spreading event led to 82 people being infected from a single hospital patient with Middle East respiratory syndrome (Mers), a coronavirus distantly related to the current virus
And in the Ebola epidemic in West Africa, the vast majority of cases (61%) came from just a tiny handful of patients (3%).
“There were more than 100 new chains of transmission from just one funeral in June 2014,” Dr Nathalie MacDermott, from King’s College London, says.
Why do some people spread more?
Some just come into contact with far more people – either because of their job or where they live – and that means they can spread more of the disease, whether or not they themselves have symptoms.
“Kids are good at that – that’s why closing schools can be a good measure,” Dr John Edmunds, from the London School of Hygiene and Tropical Medicine, says.
“Commercial sex workers were very important in spreading HIV,” Prof Mark Woolhouse, from the University of Edinburgh, says .
Others are “super-shedders”, who release unusually large amounts of virus (or other bug) from their bodies, so anybody coming into contact with them is more likely to become infected.
Hospitals treating severe acute respiratory syndrome (Sars) became a major centre of super-spreading because the sickest patients were also the most infectious and they came into contact with lots of healthcare workers.
How do they change an outbreak?
“It plays a big role at the beginning of any outbreak, when the virus is trying to get established,” Dr Edmunds told BBC News.
New infections, including the coronavirus, come from animals.
When it makes the jump into the first patient, the disease might fizzle out before it can cause a large outbreak.
But if it can quickly find its way into a super-spreader, then it gives the outbreak a boost. The same rules apply when cases are imported into other countries.
“If you have several super-spreaders in close proximity, you’re going to struggle to contain your outbreak,” Dr MacDermott says.
What will it take to stop coronavirus if there is super-spreading?
Super-spreading of the new coronavirus would not be a surprise and will not significantly change how the disease is managed.
At the moment, we are completely reliant on identifying cases and anyone they have come into contact with quickly.
“It makes that even more important – you can’t afford too many mistakes, you can’t afford to miss the super-spreader,” Prof Woolhouse says.
Is it the super-spreader’s fault?
Historically, there has been a tendency to demonise the super-spreader.
“Typhoid Mary”, Irish cook Mary Mallon (1869-1938), unknowingly passed on typhoid fever when she had no symptoms and ended up spending decades in exile and forced quarantine.
But in reality, it’s not the patient’s fault.
“We need to be careful of the language we use,” Dr MacDermott says.
“They haven’t done anything wrong, this is an infection picked up through no fault of their own.
“They’re probably afraid and need love and attention.”