Easy:
the treatment for shitting diseases. Not the
cure for the
disease; the
treatment for their
symptoms.
Dysentery and cholera are shitting diseases, which means that they desiccate your body no matter how much food or drink you put in yourself. This means that it takes away electrolytes, glucose and water for the duration of the illness. Unfortunately, the illness is usually long enough that you die of desiccation before it runs its course.
People only discovered an effective treatment (boiled salt water + honey) in the early 20th century, if I recall correctly.
Water, salt and honey were available to any shaman or army medic from the Akkadian period onward, so it's amazing that it wasn't discovered sooner.
If people had learned early on (say, ~3000 BC) that such a potion treated dysentery, there would be an immediate effect on virtually everything.
Armies historically lost most of their numbers to disease, and then mostly to shitting diseases, so this knowledge means that deaths in armies go down dramatically. Supplies of the correct materials would become valued items in any baggage train, and one of the prime targets for skirmishers attacking them.
A lot of dysentery/cholera deaths also came from people working or regularly traveling through unsanitary conditions (fields, unpaved roads, the countryside) and either getting an infected cut or ingesting infected materials (unboiled water, unclean food, etc). Thus, this remedy would also raise life expectancy among civilians, especially in the cities.
This alone means faster population growth and fewer downsides to living in a city, and thereby faster urbanization.
Faster development of science and trade follows from there.
Cholera and dysentery aren't as well-known as smallpox and plague, but they accounted for far more deaths overall than either of them, and they're up there with tuberculosis for greatest death-count of human history.
Edit: okay, why is everybody liking this post?