[Reporter: Terence McKenna]
[Reporter] In the United States, at 
the epicentre of the global pandemic
you hear strong messages about clear sailing ahead.
The virus is disappearing.
[Trump] If you look, the numbers are 
very minuscule compared to what it was.
It's dying out.
[Reporter] In recent days, the president's economic adviser Larry Kudlow
has been categorical there will be no second wave.
[Kudlow] I spoke to our health experts 
at some length last evening.
They're saying there is no second spike.
Let me repeat that, there is no second spike.
[Reporter] That's not what you hear about 
a second wave
from one of the world's foremost experts 
on infectious disease.
I think it's probably a statistical certainty
barring any unforeseen events.
[Reporter] Gabriel Leung grew up in Toronto and studied medicine at Western University.
Then he obtained postgraduate degrees
from Harvard and Hong Kong University
and is now Dean of the Medical School in
Hong Kong.
He was among the first to warn about a global pandemic
in The Lancet medical journal back in January.
And now he is warning about 
multiple waves to come.
[Leung] I think that we are adapting to a new normal
until a safe and effective vaccine becomes widely accessible
to the majority of the world's population.
And I'm afraid until that happens
we are going to be looking at perhaps more than a couple of waves of this pandemic
sweeping through our respective populations.
[Reporter] The nightmare model of what a second wave could look like
is the 1918 flu pandemic.
It was called the Spanish flu
even though it originated in the United States.
The first known cases were in Kansas in
the spring of 1918.
Then it was transported across the United States and
to Europe by U.S. troops
fighting the First World War.
In the fall, the second wave came back to North America aboard troop ships
in a much more virulent form.
Suddenly patients could start bleeding profusely from the nose and ears.
Or just keel over and die in a matter of minutes.
Philadelphia was among the hardest hit
after a victory-bond parade in September
that brought thousands to congregate in
the streets, spreading the virus.
In the end, the virus killed an estimated
500,000 Americans
50,000 Canadians
and millions more around the world.
[Leung] The 1918 h1 influenza outbreak
really in many populations
the second wave was by far worse than the first.
And I think that -- I fear might happen again.
Of course, we have no way of knowing beforehand
but I do think that it would be prudent and
responsible of governments
and public health authorities to prepare 
for such a possibility.
[Reporter] Leung sees an echo of that famous Philadelphia parade
in the massive crowds were seeing around the world
from the anti-racism demonstrations in the United States and Europe.
To the pro-democracy protests in Hong Kong.
I'm worried about any kind of mass gatherings.
Whether it's in my own town or whether it's halfway around the world
in your southern neighbour or in Europe.
Whether it's rock concerts or whether it's protests, marches
I think that we need to be very, very careful.
[Reporter] As for the prescription of how to go forward from here
Dr. Leung is categorical about a few issues.
One is the wearing of masks.
Masks are absolutely necessary but they're insufficient.
So I think that the scientific evidence, it's almost becoming indisputable now.
Wearing a mask is really a sign of respect
that you have for people you come in contact with.
The main beneficiary in fact of you wearing a mask is not yourself.
It is people around you.
[Reporter] When it comes to testing
Dr. Leung says we should be doing far more than we are doing now.
And that almost every private company should be testing its own employees.
And so I think testing needs to be ramped up
way beyond what the public sector capacity could possibly meet.
And that's why we need to enable and empower and finance public testing.
You need to also encourage, incentivize and facilitate
private sector contribution and participation.
[Reporter] He sees widespread testing as the key to resuming air travel.
Quarantining for any extended period like 14 days is a huge damper on cross-border travel
and I think that, look you know it's almost a no-brainer.
If you can then ramp up testing so that you could actually then lift
if not entirely at least some of that quarantine burden.
One element that Dr. Leung is optimistic about is vaccine development
because he says it does not have to be administered to everyone in the world
to prevent them from getting the disease.
Imagine that we get a vaccine that perhaps can take away
most of the serious complications.
It would prevent people even if they get infected
from needing to be intubated from, needing ICU care, from dying.
That would be a major advance already.
[Reporter] Gabriel Leung was among the first expert to rush to Wuhan at the beginning of this outbreak
to assess the threat on behalf of the World Health Organization.
He agrees that the role of the WHO must be re-examined
but now is not the time.
We are a marathon.
We're not in a sprint.
And I don't think that it would be helpful to try and look back
while you are still in the first half of your marathon
or maybe even not the first half yet.
So I don't know of any marathon runner who keeps looking back while they're still running.
So I think that accountability
and also transparency
and retrospective review to learn lessons is important
but we do that after the marathon has been run and won hopefully.
Not during the marathon itself.
[Reporter] When the summer ends and the cold weather returns will there be a second wave?
Gabriel Leung says the only safe course is to presume that there will
and perhaps not just one.
Terence Mckenna, CBC News, Toronto.
