In Chapter 7 of "The Great Gatsby," what does "it occurred to me that there was no difference between men..." mean?

Nick makes the comment that there is no difference "so profound as the difference between the sick and the well." He makes this observation when he sees Wilson after he has discovered that Myrtle has been cheating on him. The "shock" has made Wilson "physically sick." Nick then notes that Tom had a similar experience earlier that day when he realized that Daisy is having an affair with Gatsby, and is not "sick" at all. Nick is not referring to illness, but rather heartsickness.