Don't forget - when astronomers analysed the tail of 'comet' Elenin, they found cyanide in it.
A lot of comets do have cyanide in their makeup.
So - maybe - as we pass through the path it has taken, some people may become ill from this?

Also:

This is why YU55 appears to be spherical.
It is not a true asteroid in the accepted sense of the word.
In fact, I think it is the No 1 asteroid of them all.
It may actually be the ancient cooled metallic central core of the old planet that used to be the asteroid belt, before some calamity in the past disintegrated it.
If this is true - then this would mean this 'asteroid' would, because of the heavy element that it is made up of, (heavier than Plutonium) have a powerful gravity gradient associated with it.
When YU55 whizzes past the Earth, this strong gravity gradient will wrench at the tectonic plates of the Earth, and destabilise them.
Huge earthquakes would ensue from this meeting of two bodies.
It wouldn't have to actually hit the Earth to cause damage!