As filmmakers, we rarely get to revisit stories we've covered in the past.

But in the midst of this COVID-19 pandemic, this one particular story has been haunting me.

In 2018, we produced a short film called "Running with the Herd."

It centered around a biologist named Jack Hogg, who's been studying a herd of wild bighorn sheep on Montana's National Bison Range.

- [Jack] It's an extraordinary experience, really.

It's a chance to share an alien society.

- [Narrator] There are a few researchers, if any, that has spent as much time as Jack has studying individual bighorn sheep.

Not only does he know every individual, but he knows their entire family history.

- [Jack] The animals are habituated but they're not tame, and so it's really quite extraordinary to be able to see an animal that that really is wild.

You know, you basically just walk out and sit down and they ignore you and you get to observe their lives.

I guess I would call it a privilege.

- [Narrator] But sadly, this was not to last.

- [Jack] At the National Bison Range, we had a healthy population of 223 bighorn, but in spring of 2016 they contracted a fatal bacterial pneumonia.

Within one year, 85% had died.

- [Narrator] To really understand the impact of this respiratory pathogen, I wanna go back to our original story and introduce you to researcher Tom Besser and his team at Washington State University.

- The MBR sheep were documented to be free of the causative bacteria the year before their outbreak.

So something happened that year.

We think based on all we know about the animals and the disease, the most likely scenario is that one of the young rams from MBR jumped the fence, went on a foray looking for breedable sheep, got infected and brought it back to MBR, and there it spread like wildfire to cause the outbreak.

(sheep bleating) - [Narrator] When European settlers moved out West, they had no idea the sheep they brought with them were carrying an Old World bacterium that would prove to be deadly in the New World.

Bighorns, which are only found in North America, have no immunity to the diseases they carry.

When they come into contact with domestic sheep, the outcome can be fatal.

- This infection spreads like wildfire, and in very short time period, like a matter of a week or so, all of the animals that that animal was in proximity to become infected.

- [Narrator] For Jack's herd, the disease was devastating.

One by one, individual sheep Jack knew since birth died.

Every single one had a name and life story.

- [Jack] Of the total 220 individuals that were alive at the start of the epidemic, 37 survived.

- [Narrator] So tell me what we're looking at here.

- While you watch this clip, passing through the frame are all survivors of the 2016 epidemic.

The entire population at this time of bighorn on the National Bison Range.

They're in a very tight cluster.

All of the rams and all of their ewe survivors are present in the group.

This time of year in late summer, that's very unusual Included in the group is the last lamb standing from this year.

This lamb was at this time, sick with pneumonia and soon to die.

That told us that there was still in the population, an adult that had recovered more or less from the pneumonia, but was still a carrier of the pathogen.

- [Narrator] With no lambs surviving and a Typhoid Mary still loose within the herd, the decision was made to capture and test the remaining bighorn sheep.

- If we get nothing else, we need to get the nasal swabs 'cause this is where they'll look for DNA of mycoplasma.

The result was that we had one female carrier in the population and she was apparently the source of infection for the lambs.

So we prepared to remove her, but she was killed evidently by a predator before we had a chance to do that.

- [Narrator] For the first time in four years, the bighorn sheep at the National Bison Range are free and clear of the disease.

- Seven lambs survived the gauntlet of early predation.

None of them contracted pneumonia and all of them reached their first birthday.

There was a feeling, I mean certainly I had the feeling that we were kind of getting to the end of the tunnel.

- [Narrator] But Jack and his team would soon discover this was not the end of the problems for the herd.

- Adult mortality remains high and females are having reproductive problems.

We don't understand why, but the lesson seems to be that a full recovery is a process that can take years if it happens at all.

Much like we're beginning to suspect with COVID-19 in humans, which is another potentially severe disease caused by a novel pathogen.

- [Narrator] Across North America, bighorn sheep have been impacted by disease.

You could say they've been facing their own pandemic for the last century.

- If there's one message, it's that things like this, it's catastrophic and perhaps you read about it in the newspaper, you know, about a die off, but then that's the end of it.

The public assumes that the population has recovered.

That's so far from the truth and the public has to stay vigilant.

They have to stay on top of this problem and recognize that it's ongoing, it's continuing, and more has to be done.

More has to be learned and more management action has to take place.

- [Narrator] It's a story that sticks with me because it makes me wonder what the road to recovery will look like for us.