There is an early Iron Age civilization spread out along the riverbanks of an enormous tropical river (think Amazon or Congo). There are small villages spread out along the banks and islands of the great river and its tributaries. Larger cities, floating or on high ground, of up to 100,000 people are built at critical river junctions and places where caravan routes cross the rivers.
Various infectious diseases are obviously a large problem for this civilization. A simple expedient that would be very effective to prevent many diseases would be mosquito netting. To develop an advanced civilization, I imagine that these people developed effective mosquito countermeasures from the Neolithic, early Farming period. As agriculture develops, this civilization is able to move from scattered bands to permanent villages to densely populated floodplains and cities. While making this transition, they must develop defenses against mosquito-born diseases to keep them from all dying of malaria by the time they live in densely populated cities.
What mosquito countermeasures would be developed alongside agriculture using products available in the rainforest? Eventually this civilization reached the Iron Age and was in trading contact with the nearby savanna and highlands (think the Peruvian Andes or African Rift mountains). Bonus points for discussing improvements would be possible with additional technology and trading resources.