The COVID-19 coronavirus is a virus that jumped from animals to humans in late 2019.
It belongs to the family of coronaviruses, which have long been known to cause human disease.
But until late last year, it had never been seen before in humans.
It makes us sick because it can attack cells in our lung, specifically the cells that help keep our lung open and produce substances called surfactants that help us in our breathing.
That's why people get very severe lung infection or pneumonia when they're infected with this virus.
And the transmission from person to person occurs via droplets, and it occurs from one lung to another person's lung.
The virus can, beyond the lung, the virus can enter cells via lock and key mechanism.
The lock are the protein studs on the viral surface and the key is a very specific receptor on cells that allows it to penetrate inside the cell and cause damage to the organs that express the lock.
And so beyond pneumonia, patients who get severe COVID-19 disease can also have complications or viral infection of the heart and of the kidneys.
And that causes some of the very severe illness that we see in the hospital today.