cheese
Explanation
This comic is a simple gross-out gag built on the well-known fact that many fine cheeses smell terrible.
The single large panel shows a man standing in a dark doorway, covering his nose and mouth in horror. He exclaims: "Jesus H Christ! Oh my God. It smells like a thousand people had diseased feet and they all got cut off and left in this basement for an entire summer." The description is deliberately extreme and revolting, painting a picture of the worst possible smell imaginable.
The caption below reads: "Later, to Steve's delight, it turned out to be a cheese room."
The humor comes from the reversal. The setup leads the reader to expect something horrifying -- a crime scene, a biohazard, or some other nightmare scenario. Instead, the punchline reveals it is simply a room full of cheese. The extra twist is that Steve is delighted by this discovery, implying he is a cheese lover. What initially seemed like a nightmarish stench turns out to be, for a cheese enthusiast, wonderful news.
The joke plays on the genuine overlap between the smell of aged cheeses (particularly washed-rind varieties like Limburger or Epoisses) and the smell of feet, since both involve similar bacterial processes. The comic simply takes that resemblance to an absurd extreme.