time
Explanation
The Joke
A patient visits a doctor, remarking that going through a terrible divorce is tough but taking comfort in the old saying "time heals all wounds." The doctor responds with surprise: "What? Really?" The next panel, labeled "LATER," shows a newspaper headline reading "TIME INEFFECTIVE AGAINST LEISHMANIASIS" with a subheading about trial results showing that the passage of time did nothing to heal the parasitic disease. The doctor apparently took the folk saying literally and conducted an actual clinical trial testing whether the mere passage of time could cure a tropical disease.
The Humor
The comedy comes from the deliberate misinterpretation of a metaphorical saying as a literal medical claim. "Time heals all wounds" is meant as emotional reassurance -- that grief and heartbreak will fade over time. The doctor, however, treats it as a testable scientific hypothesis and goes so far as to run a clinical trial. The choice of leishmaniasis (a real parasitic disease transmitted by sandflies) as the test case is particularly funny because it is so far removed from the emotional context of the original saying. The joke satirizes both the overly literal scientific mindset and the absurdity of folk wisdom when taken at face value.
References
Leishmaniasis is a real disease caused by Leishmania parasites, spread through the bites of phlebotomine sandflies. It is endemic in parts of the tropics, subtropics, and southern Europe, and without treatment it can indeed be fatal -- making "time" a spectacularly poor treatment option.