From a slight fever to pneumonia and heart damage, the symptoms of COVID-19 can vary wildly from one person to another.

But the patients who land in the care of Dr. John Granton are typically on the most severe end of the spectrum.

Granton, an intensive care doctor at Toronto General Hospital and head of the respiratory department at Mount Sinai and University Health Network, is among Canada's doctors fighting the disease on the frontlines of the pandemic.

Here's what he's learned about the disease so far.

The following interview has been lightly edited for length and clarity.

Q: What does a severe case of COVID-19 do to the body?

A: It's early days but we do have experience with similar coronaviruses, SARS being one and MERS as well. This is a new virus, however, so we're learning on the fly.

What we do know is that it can range in severity from being very mild and people not recognizing that they have the condition to people who have to become hospitalized because they're sick. And in some instances people become critically ill and have to be put on life support.

Q: What do those severely ill patients experience? What is the damage that's being done to the body?

A: People most commonly present when it's severe. They develop a severe fever, a severe cough and the important one we worry about is shortness of breath.

When we see patients in the emergency department, we see that they're short of breath, that their oxygen levels are low and that they require additional oxygen to help their lungs to work properly as the virus is infecting their lungs.

For some patients, unfortunately, the amount of oxygen you can provide outstrips the resources on a regular floor and they have to go on a breathing machine to support their breathing and they have to be kept sedated and kept on life support until hopefully they recover from that illness.

We're also recognizing that this virus oddly seems to involve the heart in some instances and can lead to dysfunction of the heart so it's not pumping properly. In some instances, unfortunately, that has taken the life of some patients due to not only the lung problem but also the heart problem.

Dr. John Granton, the head of respirology at University Health Network and Mount Sinai Hospital. (Zoom)

Q: What happens to someone's lungs or body that makes them feel shortness of breath?

A: It sounds like an easy question but we don't always understand why people feel shortness of breath. There's probably some irritation of the lungs, as well as not getting enough oxygen. So what it feels like is the things you do day-to-day feel more difficult. It can feel like you're running, you're panting doing minimal things.

Q: What's the outlook for someone who has to be put on a respirator or on oxygen?

A: At present, we know no treatments for this particular virus, there's no vaccine to prevent it and there's no treatment specifically for it.

What we do in critical care is we kind of cradle people through their illness, we support them on life support until their bodies can recover; their lungs can heal, their heart gets better and the other organs that may have been suffering secondary damage can recover to the point where they can be liberated from those machines.

We really don't understand why certain people get more severe disease than others. - Dr. John Granton

And so therein lies a bit of the problem. Because there's no specific therapy and we rely on the body's recovery, that means that people who have underlying medical conditions, people who are older … may be more susceptible to developing a severe pneumonia or heart problem.

I don't want to frighten people, but people have been trivializing this saying 'I'm young, I'm not going to get sick,' but half of the patients in intensive care units are under the age of 60 with this condition.

Nobody's immune from this particular illness.

Q: Why is it that some people seem to be more susceptible to this? What do we still need to learn about the virus and how it works?

A: Most of us will not get this virus, let's be clear. And most of us who get the virus will be fine. This is an uncommon condition but still it's coming at us in a wave that's causing problems and stress on the system.

We really don't understand a lot of things. We really don't understand why certain people get more severe disease than others. Is it a property of the virus? Or is it a property of the person? We all have different genes and different responses to infection.

This is not unique. We know some people can get very trivial infections from the flu, and other people get severe flu and end up on life support, it's really not different in that regard.

[We] don't understand why some people have severe disease and others do not. But again, if you have an abnormality in your health or you happen to be older, your immune system is not as robust as it used to be. As a result, your ability to fight infections and recover from those illnesses is diminished.