Essentially, the lesions that the characters get in the novel Feed are the product of the toxic atmosphere that permeates the environment of this dystopian society. In Feed, society is entirely dependent on their scientific progress. However, like with all things progressive, there must be balance.
In Feed the key issue is that there is no system of checks and balances and, for that reason, the people and their technology have intertwined so much that they view technology almost as a natural resource.
The lesions are produced by radiation and by accidents caused by the different industries. The contamination is everywhere and it is slowly rotting the people away. As such, the lesions also become fashionable after a while, which is indicative of the way that the people are oblivious of how bad things really are. Hence, the lesions, like the feed, like the radiation, and like all the other things are (in the minds of this society) a natural occurrence; an expected consequence of the benefits of technology.
And the feed was pouring in on us now, all of it, all of the feednet, and we could feel all of our favorites, and there were our files, and our m-chatlines. It came down on us like water. It came down like frickin' spring rains, and we were dancing in it.
This is the extent to which this society has almost hybridized the natural and the artificial. The lesions are the sad product of an unbalanced society who has allowed technology to, literally, eat away its humanity and common sense.