In the world I'm envisioning, an individual has been accidently sent to a sort of alternate history version of the world, one that is at roughly feudal level of tech/culture, but with the nations divided up differently than in our history. The Traveler didn't plan for this transition and thus didn't have time to prepare for it.
After arriving, he earned some interest from the king of a nation (impressing the king with the laptop the Traveler carried before the battery inevitably died) and promised to try to help improve the nation with his future knowlege, but his inability to produce any quick & obvious feats with his knowledge leaves the traveler with limited support from the king.
Around this time, another neighboring nation has a major disease start to spread, and it looks like it could become a full blown epidemic. Both his king and the neighboring king are worried, and so the Traveler's king asks the traveler to use his future knowledge to fix the disease (the Traveler had sold himself especially on his promise to help with lowering disease via use of sanitation techniques).
The Traveler is very knowledgeable about evolution & genetics, providing him some tangental knowledge about disease beyond basic high school biology, but he is not a doctor or deeply trained in handling of disease. His first attempt to help is sending basic 'best practices' he is aware of for working with disease, like quarintene, sanitation, use of face masks, and boiled water. He also sends a rather long 'questionare' to the foreign king which he asks the king to send to numerous city healers which asks a number of questions about frequency of disease, traits common to those infected, and targeted questions trying to determine what method of spreading the disease may use. The answers he eventually get back point to one commonality: areas with stagnant water or which otherwise would be prone to high number of mosquitoes have far higher rates of infection. His questions targeting other potential plague vectors haven't shown any other likely plague vectors.
So having a theory as to how the disease is spread, it's time to send advice on how to get rid of the plague vector. The questions I have are twofold. What kind of advice (other then the obvious “mosquitoes are bad” and “try to avoid stagnant water”) can he give to combat the mosquitoes’ spreading of the disease? How significant an impact can this and the earlier basic sanitation advice have in stopping the spread of the disease? Keep in mind that he hasn't yet proven himself to the foreign king and thus the king is not yet confident how much money/effort he is willing to commit to following through on the advice of a stranger.
If the Traveler's advice does help, how obvious to the kings will it be that the advice was the cause of the disease loosing momentum?