The chances of a COVID-19 outbreak in the UK have been significantly raised by the first "community" transmission of the virus.
Until now doctors have identified suspected cases by testing people who have returned from certain high-risk countries and have flu-like symptoms.
It will be much harder to identify infected individuals who have not been outside the UK.
There are many other causes of coughs and fevers at this time of year and there simply would not be the capacity to test everyone for the virus.
The concern for public health teams is that there may now be a significant chain of transmission in the UK.
The individual in Surrey is unlikely to have considered that their symptoms were due to the coronavirus, so they may not have sought medical help straight away with a high chance of spreading it in the meantime.
Even more of a worry is the source of the virus hasn't been identified.
Who gave it to the patient in Surrey? How many others have they infected? And have they in turn also passed on the virus?
Contact tracing in those circumstances is much more challenging.
Patient 0 - the original source - has never been identified in the Italian outbreak.
It's likely they only had mild symptoms and kept going with everyday activities, seeding an outbreak that remained below the radar for several days.
Just over a week ago Italy had just three reported cases.
Now it's over 800 according to official statistics and scientists believe there are many more still undetected.
Whether there is a similar explosion of cases in the UK depends on the success of public health doctors in shutting down every chain of transmission.
And the longer it takes, the harder it will be.