Nick recognizes that Tom and Wilson are having the same reaction to discovering their wives are having affairs. Nick writes that for Wilson, "the shock had made him physically sick." It then strikes Nick that Tom had made a "parallel discovery less than an hour before," about Daisy and Gatsby. It is here that Nick writes, "there was no difference between men, in intelligence or race, so profound as the difference between the sick and the well."
This is important, because it flies into the face of the racial and class categories that Tom lives by and his preoccupation with the importance of keeping the "Nordic" races pure. It is unclear whether or not Tom would consider Wilson a Nordic, but he clearly considers him an inferior who can be treated with rudeness and contempt, and whose wife is fair game for him. But while Tom might primarily see the gulf between himself and Wilson, Nick perceives their common humanity: it is what people suffer, what they experience, their emotional makeup, that makes them alike or different, not a superficial racial designation or even their level of intelligence. The two categories Nick divides people into are based on emotional health or lack thereof, and his flash of insight is that these transcend other differences.
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.