Iago is referring to the plan that he has been setting for Othello. He says "Work on, <span>My medicine, work! " which shows that he's trying to make sure that the hints and comments he has dropped will take hold in Othello's mind so that he believes Desdemona and Cassio are having an affair. </span>
This was at the begining of the age of the industrialisation, factory work was very harsh, workers had to live in a very small and crowded space, many weren't insured and didn't get enough money for working 12 or more hours a day.