"We have never seen anything like this in our lives" - you will hear this saying again and again in the mouths of doctors who have treated patients with Kovid-19.
These British doctors - many of whom have worked in the intensive care units of hospitals - were aware of the new virus that had spread from China. Many of them have heard and read the experiences of their colleagues in China and Italy.
They knew it was only a matter of time before the disease reached Britain.
But when the coronavirus came to the UK, even the most experienced doctors in the ICU were surprised to find the virus.
Most patients with coronavirus had mild symptoms. Some may have had no symptoms at all.
But the thousands of patients whose condition had reached critical levels - doctors found while treating them - Kovid-19 is actually a surprisingly complex virus.
Here is a description of what doctors have learned and what they have not yet learned about how Kovid-19 attacks the human body.