In November 2016, on a very hot day in Melbourne, Australia, a thunderstorm rolled into town.

And suddenly, thousands of people found themselves struggling to breathe; a strange outbreak of asthma, sparked by a storm. In just five hours, paramedics were called nearly 2,000 times.

By the end, nine people had died, and hospitals had treated 8,500 patients. So, what happened? Well, thankfully, an event like this is rare, but it has happened before.

It’s known as thunderstorm asthma. Several other instances of thunderstorm asthma have been documented in the US, Canada, England, and Italy. This latest event, though, was the most severe we know of.

Previously, only one person had died from thunderstorm asthma, and the largest attack affected just a few hundred patients. Scientists aren’t exactly sure what causes thunderstorm asthma, but the best guess is that a storm picks up pollen and other allergens, and then concentrates them so that people breathe in a lot of them in a short amount of time. In people who are allergic to these things, inhaled pollen and mold can trigger an immune response that causes airways to close, kicking off an asthma attack.

But the problem isn’t just that a bunch of allergens are blowing around. Normally, most pollens are too big to get into your lungs, even if they’re blown right into your face. A storm, though, can change that.

Updrafts are thought to pull pollen up into clouds, where pollen grains come in contact with water. And when that happens, the pollen bursts open, with each grain releasing hundreds of tiny granules. Windy downdrafts can deliver these microscopic bits of pollen back to the ground, where they can be inhaled -- and this time, they can go deep into your lungs. A similar process is thought to happen with the tiny spores produced by fungus, which can get kicked up in a storm.

Now, even though it’s called thunderstorm asthma, it’s not just asthmatics who are at risk. A recent survey of the Melbourne victims found that 40 percent of people who experienced symptoms hadn’t been diagnosed with asthma before. Instead, an important factor in these events seems to be whether you’re allergic to whatever kind of mold or pollen that a storm is stirring up at the time.

That means anyone with hay fever, or allergies to seasonal pollens, might be at greater risk for thunderstorm asthma. One reason scientists think the Melbourne case may have been so severe is that the culprit pollen was ryegrass, which many people have an allergy to, and which was especially abundant at the time. Since about 20 percent of the world is sensitive to some sort of grass or tree pollen, part of the danger with thunderstorm asthma is simply that a lot of people might need help at the same time.

As was almost the case in Melbourne, there might not be enough ambulances to get to everyone fast enough. Complicating things more, these attacks can catch many people off-guard, especially non-asthmatics. Unless you have asthma already, most people with hay fever just get runny noses or scratchy eyes, but don’t really have trouble breathing.

So, those people don’t have the medicines they need on hand, like a rescue inhaler. While these are freak events, and not something you need to worry about every day, some experts say they’re likely to become more frequent with climate change. In many parts of the world, hotter temperatures are predicted to increase the number and severity of storms.

And extra carbon dioxide in the atmosphere could spur plants to make more pollen, or even more potent pollen. So, if a storm seems to be headed your way, and the pollen count is high, you might want to check to see if you have your inhaler. Just to be sure.

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