Every time there is a new outbreak, there's a roll of the dice.
That's because Covid-19 is so random in who it infects and how easily.
The latest episode of our ongoing battle with the virus stars an extremely unfortunate Defence Force serviceman, who passed it to an Auckland student he doesn't know and has never met.
Investigations are ongoing, but following extensive interviews with both of them, they don't appear to have interacted.
Even if the virus jumped from him to her in a chance encounter, it would defy conventional wisdom that Covid-19 is normally transmitted between people who spend 15 minutes within two metres of each other.
This appears to be the kind of rare, impossible-to-anticipate transmission that no lines of defence could prevent, short of having everyone in personal protective gear and staying two metres apart from each other at all times.
All we know for sure at this point is that they were in the same area at the same time.
On Thursday last week, the serviceman had lunch at the Mezze Bar which, according to Google maps, is 82m from the A-Z Collection shop where the student worked.
Right next to the shop are two public toilet cubicles. Did they use the same cubicle shortly after one another?
Transmission via air particles is possible, as is surface transmission, though it is rare.
The latest case caused enough concern for Prime Minister Jacinda Ardern to immediately fly to Wellington yesterday after being told of the infection.
The last time a case with no link to the border or a known case popped up, Auckland went into level 3 lockdown and there were 179 cases across two regions.
For this latest case, lockdown remained possible, or even probable, until a source was determined. The immediate restrictions put on the CBD were quasi-level 3.
If bad luck had prevailed, Covid could have spread via a passenger sharing the student's Uber driver, a diner sitting near her at the Red Pig, or any of the dozens of people living at the Vincent Residences who might have shared a lift button or a rubbish bin lid.
(It still might have spread in this way, though the chances are slim.)
But then the dice started rolling our way. The student lived alone. She only had a few workplace contacts. She hadn't been to the supermarket. Neither she nor her three close contacts had left the CBD.
Combined with the timeline of infection, it seems likely that he infected her, meaning there are no missing links in the transmission chain to unearth.
A new mystery community case, then, and a lifting of CBD restrictions in just over 24 hours.
It was suggested to Covid-19 Response Minister Chris Hipkins that this was luck masquerading as competence.
And, as epidemiologist Sir David Skegg said, there's always been a "modicum of good luck" in New Zealand's world-leading Covid-response.
Relying on luck is never a good tactic, and the Government obviously doesn't do that. We have lines of defences to prevent Covid-19 from spreading.
But public health experts have been calling for those to be strengthened, including mandatory mask-wearing on public transport and flights at level 1 and compulsory QR scanning.
Hipkins has resisted, apparently because the public's goodwill towards the Government response might crumble in the face of such an apparently over-the-top order.
But the mood to keep the faith is always strongest in the face of a new threat, so Hipkins changed his mind on face masks for Auckland - with Ardern's backing - and other centres are likely to follow.
He is still looking into the legalities of mandatory scanning, how it could be enforced, and whether it should only apply to high-risk places, such as events or hospitality venues.
Even if it can't really be enforced, an order would still push up the number of scanners and scans.
Only one in six people registered with the Covid Tracer app are using it. With that kind of abysmal ratio, our luck will eventually run out.