Riddle poem

A horror that kills thousands unnamed,
The truth of this is not proclaimed.
We know not of it's dangers, it's potential dismissed,
You cannot imagine it's numbers in midst.

A poison that is found in every lake,
Such, so that it's found in every cake.
From stream, to river, to ocean and sea,
It can be found everywhere, in even a pine tree.

You may not know it, but aujourd'hui,
You will certainly use it to a tolerable degree.
With all aforementioned, you must have in mind:
"Whatever it is, our fates are intertwined."

What am I?