To explain my interpretation a bit more, I believe he is talking about the mind viruses that control everyone. He is immune to them - aware of them, not subject to them like everyone else.
The poison is the mind control they run on us all perpetually.
Similarly, Radiohead talks about the poison too. And the top of the pyramid.
Try to understand
...
Because you bit me, bit me
Bit me, oh
I got the poison, poison
And now I want more
If you are on the top
Then it is a long drop
The pyramid is power
We're changing by the hour
If you are on the top
Then it is a long drop
If you stare into the dark
The dark will stare back
Back into your soul
I'm taking you down
I'm taking you down
I'm taking you down, when I go down
I'm taking you down
I'm taking you down
I'm standing in the hall
I'm puking on the wall
Yeah, because you bit me, bit me
Bit me, oh
The poison, I got the poison
I got the poison now
Yeah, I got the poison