Though Death in Venice doesn't reveal much about the epidemic that takes Aschenbach's life, we do find out that the disease is cholera. And you know what? That hint turns out to be pretty important. Let's take a look:
For several years now Indian cholera had displayed a growing tendency to spread and migrate. Emanating from the humid marshes of the Ganges Delta, rising with the mephitic exhalations of that lush, uninhabitable, primordial island jungle shunned by man, where tigers crouch in bamboo thickets, the epidemic had long raged with unwonted virulence through Hindustan, then moved eastward to China, westward to Afghanistan and Persia, and, following the main caravan routes, borne its horrors as far as Astrakhan and even Moscow. But while Europe quaked at the thought of the specter invading from there by land, it had been transported by sea in the ships of Syrian merchants and shown up in several Mediterranean ports simultaneously. (5.32)
Cholera is described here as an essentially exotic disease, and one that has invaded European lands. But with the narrator's evocative descriptions of the "humid marshes of the Ganges Delta," the "mephitic exhalations" of a "primordial island jungle," we're also reminded of Aschenbach's initial "enigmatic craving" for far-off places, "a tropical quagmire beneath a steamy sky" (1.6), inspired by that strange guy he sees in the Munich graveyard. In other words, the exoticism of cholera is aligned with Aschenbach's own cravings for something, well, different.
And there's more. The "stranger god," the symbol of Aschenbach's repressed, so-called primitive desires and impulses (more on that elsewhere in this section) is, as the name implies, a stranger. Just like cholera. And just like cholera, it is associated with base, animalistic behavior. Cholera comes from a place "shunned by man, where tigers crouch" and the stranger god dream is filled with, amongst other things, humans giving themselves over to animals. In both cases, animals rule.
And because of these parallels, cholera also represents Aschenbach's desires—and importantly, it very concretely adds an element of disease to them. He in infected by his longing, governed by it the way cholera takes over a body. Sigmund Freud (more on this in Aschenbach's analysis in the "Characters" section) famously described the unconscious mind as a "dark continent," awaiting the exploration of scientists and therapists, and Mann's description of cholera as originating in the uninhabitable jungles of India seems to give Freud's words a literal interpretation.