He says that land speculation is a fever and the spectators are patients in The Devil and Tom Walker. What is he saying about the love of money?

The moral of the story, "The Devil and Tom Walker" is that greed will land a person in hell, which is what happens to the very greedy Tom Walker.  The deal Tom made with the devil was for money and the means to that money was a business that Tom ran and the devil created in which Tom lent money to people who were land speculators, among others.  This was when there was lots of wilderness country and people bought large amounts of land hoping to sell of portions of it at a good profit.  In order to get the money to buy the land, people turned to lenders and Tom Walker was a lender who charged an extremely high rate of interest and foreclosed without mercy for non-payment.  To say that land speculation was a fever means that once the idea of buying land and then selling it to make a profit got in someone's head, they couldn't let go of the idea.  That speculator was sure he was the one who was going to make a huge fortune by subdividing his land.  Sometimes, however, there would be no buyers - the land was not good land or there was just so much of it for sale.  The determination and desperation to sell the land made people to make foolish decisions such as to borrow money from lenders like Tom who were no more than loan sharks. That's why the speculators were "patients" - they were often sick with greed.