International air travel is likely to be one of the last things to be freed up as the COVID-19 crisis comes to an end.
Letting people travel between hotspots of disease and areas clear of it is a surefire way of continuing reinfection.
Flights between areas relatively free of the virus will start soon. New Zealand Prime Minister Jacinda Ardern is already talking about flights across the Tasman Sea.
But the full international network will be a harder task.
What are the options?
Many have been mooted: COVID-19 "passports", certifying the bearer as having tested negative for the virus; removing alternate seats in aircraft; and strict isolation at both ends of the journey.
And at the airport, hygiene has already been ramped up - Canberra Airport has just unveiled its thermal imaging to test passengers for symptoms of fever.
Just as security was transformed after 9/11, so precautions against germs are likely to increase as you go through security.
Security will mean security against terrorism and security against infection.
What's already happening at airports?
People are still flying.
The International Civil Aviation Organisation, part of the United Nations, reckons that this year the number of passengers will be down by between 39 per cent and 72 per cent, depending on how soon the virus is brought under control - but that still leaves many people travelling.
Social distancing and hand-sanitisers are now de rigueur at airports.
Beyond that, Hong Kong Airport is now testing a full-body disinfecting machine. Passengers go into a chamber for 40 seconds and are sprayed. The airport says: "The interior surface of the channel is equipped with antimicrobial coating which can remotely kill virus and bacteria on human bodies and clothing."
Does that detect the virus in infected people?
It does not. It just ensures that people who may have picked up the virus on their clothing or hands can't pass it on.
One idea is to have "immunity passports" so that people who have already recovered from the disease can be certified as non-infectious.
But we are some distance from that. Firstly, too few people have been infected to provide enough passengers for air travel to be viable again.
Secondly, testing would need to be available to all passengers, pre-flight and with confidence that they hadn't picked up the virus since.
And, thirdly, it is not completely certain that a person who has had the disease can't be reinfected.
Why can't we just quarantine people?
That is possible and it already happens in Australia. People flying in from other countries must be isolated in a hotel at the point of arrival which usually means Sydney or Melbourne.
The taxpayer foots the bill but it won't be a holiday. Quarantine means staying in your room, with food left outside the door.
And what about passengers who live in, say, Canberra, Illawarra, Newcastle, Tasmania or Tamworth. They would not be able to proceed home for two weeks.
The United Kingdom is also thinking of bringing in 14-day quarantine.
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The idea there is that incoming passengers would have to stay at a designated address. They would be subject to unannounced inspections with big penalties for not being there.
How the passenger would get from the airport to the address hasn't been specified.
What changes might work on the plane itself?
"In-flight janitors" may become a new job description.
Just as after 9/11, American airlines mooted having armed, plain-clothes "marshals" on board, so a new post for someone to perpetually clean and disinfect surfaces in-flight might be an option - if only to provide reassurance.
Staff need protection, too. Korean Air says: "Cabin crews on all flights are wearing goggles in addition to masks and gloves, while protective gowns are being provided on medium- and long-haul inbound flights as overseas COVID-19 cases continue to increase significantly."
The idea of leaving empty seats between passengers is unlikely to work - even if you discount the idea of people congregating near toilets and clambering over each other to get there.
How does economics kick in?
The problem is not just one for doctors. Airlines have to be able to make enough money to fly passengers even if the health checks are robust.
The empty seat idea won't fly because airlines need full planes to make money.
We can't all afford to travel in isolation in first class.
On top of that, the most lucrative routes are those between big cities - and big cities are the hubs of the virus. New Zealand and Iceland are in the lead in cutting out coronavirus but there's not much demand for travel between them.
Unlike terrorism, COVID-19 will pass, or so we assume hopefully.
The best estimate is that a vaccine will be available in a year to 18 months - though it is not certain that a vaccine can be developed. Not all diseases are preventable.
Airlines and airports may take the optimistic course and calculate that the crisis will pass so major investment is just not worth it.
So what is likely to happen?
About 130 countries have closed their borders. The degree of closure varies. Australia and New Zealand are allowing back citizens and permanent residents, but with strict quarantine on arrival.
The likelihood is that borders will be reopened to airline passengers on a case-by-case basis. As the International Air Travel Agency puts it: "For aviation, a key challenge will be countries removing restrictions at a different pace according to national circumstances."
Each country would need to trust the statistics and the measures at the other end.
Internal flights will pick up as (and if) the virus recedes in Australia.
And New Zealand is nice.
