I remember that day so well, I'll never forget it.

When did you first realize the potential impact of COVID-19?

For infectious disease doctor Monica Gandhi,

it was while she was in a meeting with her colleagues earlier this year,

when the reality really set in.

We had our division meeting and we're all of course in person in a crowded auditorium.

And at the end we honored the ophthalmologist in Wuhan who had died

and we had a picture of him and just said,

how sorry we were that this new infection had arisen.

She's talking about Dr. Lee Wen Liang,

the 34 year old Chinese doctor who is one of the first people to raise the alarm

about what he was seeing at the hospital where he worked.

This was before COVID became a pandemic.

He died from the virus in February despite being young and presumably healthy.

Even now that remains one of the mysteries of this virus.

When people become infected, some people come down with mild symptoms or none at all.

While others have to be hospitalized, even die.

We know there are risk factors, we know your age, pre-existing conditions, they all play a role.

But still, why does this coronavirus hit some people so much harder than others?

On today's podcast, I'm going to talk to Dr. Monica Gandhi about one possible hypothesis.

Viral Dose.

I'm Dr. Sanjay Gupta, CNN's Chief Medical Correspondent.

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