Yellow fever is a disease associated with tropical areas, so this passage suggests that Dobbin has been fighting out in Britain’s colonies. This passage implies, without apparent satire, that Dobbin’s military experience in the colonies is evidence of his good character, suggesting that on some level, Thackeray may have approved of Britain’s colonial enterprises. Today, most writers and historians judge Britain’s colonialism more harshly, in part because of the violent methods used to control the colonies.